A Systematic Literature Review Example on Impacts of Whale Stocks Decline on Marine Production

Overview of this Systematic Literature Review Example

This systematic literature review example, conducted by Best Dissertation Writers, examines the impact of commercial whaling on whale stocks and the subsequent effects on marine production. The review synthesizes evidence from five studies sourced from the Biological Science Database and Environmental Science Database, highlighting three key roles of whale stocks in marine ecosystems: productivity enhancement, carbon sequestration, and nourishing deep-sea environments. The findings reveal that whales contribute significantly to marine production by acting as both consumers and prey, facilitating nutrient transport, and providing essential resources through whale falls. The decline in whale populations poses a serious threat to the efficiency and sustainability of marine ecosystems.

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Abstract

Commercial whaling has negatively impacted the volumes of whale stocks across different marine ecosystems. There is increasing volumes of evidence showing that apex predators play a central role in ensuring sustainability natural ecosystems. However, limited research has been conducted in the context of marine ecosystem with specific focus on the role of whales. Therefore, the primary aim of this study was to assess the negative impacts of whale stock decline on the marine production. A systematic literature review approach involving synthesis of evidence collected from previous studies about the role of whale stocks on marine production was adopted. The five studies included in this review were derived from two databases; Biological Science Database and Environmental Science Database. Thematic analysis of evidence from the selected five studies led to the identification of three themes describing the roles played by whale stocks in the marine ecosystem; they include productivity enhancement, sequestering carbon and nourishing the depths. The present review has established that whales contribute to the marine production through different strategies such as acting as consumers and prey in the food chain, whale falls and acting as transport channels for nutrients and other limiting resources such as carbon in the marine ecosystem. Therefore, decline in the whale stocks would negatively impact the quality and efficiency of marine production hence the need to formulate and implement strategies for protecting and ensuring recovery of whale populations.

Keywords: whale stocks, whaling, marine production

Chapter One: Introduction

Research Background

The era of large-scale whale hunting lasted for approximately 400 years, that is between the 17^th century to mid-1980s (Hoareau, 2020). During this period, whale hunters mercilessly killed, exploited and depleted one species to the other. When though the precise number of whales that were killed during this period are not known, evidence from the study by Mamzer (2021) shows an estimate of at least 3 million whales being killed in the 20^th century specifically for their valuable oil. In 1986, the International Whaling Commission (IWC) implemented the global moratorium on commercial whaling (Wangn et al., 2018). The move was specifically motivated by rapid decline in the whale population with the most affected whale species being the blue whales from Southern Hemisphere which decreased by 95% as a result of hunting and marine environmental changes (Alamri, Alsariera and Zamli, 2018; Hoareau, 2020). Despite the fact that the IWC’s moratorium still remains active, scientific and commercial whaling continue, with countries such as Norway, Japan and Iceland killing more than 45000 whales since 1986 (Early and Preble, 2020; Mamzer, 2021). The models for population dynamics developed by the IWC primarily require historical records, current abundance and approximations for the intrinsic rates of whaling practices, all these areas are subject to substantial uncertainty.

The whaling practice in general has caused significant reduction in the number of whales in the marine ecosystem. For example, the baleen whales and sperm whales have been systematically hunted during the past two centuries, making these whale populations vulnerable to extinction (Baker and Clapham, 2020; Saidala and Devarakonda, 2017). Such exploitations have been established to almost eliminate the whole trophic levels of the marine ecosystem, specifically the southern hemisphere. Despite the increasingly being formulated and adopted protection policies in the recent years, most of the whale populations such as the North Atlantic right whales (Nøttestad, Fernö and Axelsen, 2018), North Pacific right whales (Baker and Clapham, 2020; Lopez and Lopez, 2017; Orams, 2019), southern hemisphere blue whales (Aoki et al., 2017; Pitman and Durban, 2019) and western Pacific grey whales (Aoki et al., 2018; Sayed et al., 2016; McInnes et al., 2020) have not shown any significant sign of recovery. A previously ignored consequence of whaling was that it prevented whales from achieving their evolutionary purpose within the marine ecosystem (Alamri, Alsariera and Zamli, 2018). Every species in an ecosystem has a specific role to play in ensuring continued sustainability of the ecosystem (Early and Preble, 2020; Wang et al., 2018). Within a properly functioning ecosystem, all species often participate in a symbiotic relationship in order to maximize productivity and abundance of the nature’s parameters. Historical reconstruction of whale populations and dynamics before, during and after the exploitation periods are key to the restoration of the marine ecosystem as well as for developing most sustainable commercial whaling practices to be adopted in the future.

Even though Lopez and Lopez (2017) proposed the use of population genetic parameters to develop independent estimates of whales’ historical demography, Nøttestad, Fernö and Axelsen (2018) and Orams (2019) on the other hand reported that such parameters are often characterised with a lot of uncertainties, such as general rate of gene flow and mutational substitution. As reported in the study by Saidala and Devarakonda (2017), the genetic and demographic estimates of pre-exploitation abundance significantly vary in the context of magnitude hence suggesting vastly different baselines for assessing the whale population recovery rate.  Therefore, there is need for more extensive research with emphasis on assessing and reporting key strategies that can be used for enhancing whale population recovery and its impacts on the marine ecosystem.

Justification of the Study

Whale population has been widely hunted to extirpation and near extinction in most of the marine ecosystems around the globe (Aoki et al., 2017; McInnes et al., 2020). Based on the fact that whales form both top predators and plankton feeding megafauna (Aoki et al., 2018; Pitman and Durban, 2019), the present study focuses on exploring the possible knock-on effects that whale stock reduction can have on the sustainability of marine ecosystem, especially the marine food-webs. Therefore, the present study would assess and report the impacts on whaling on marine ecosystem sustainability, including effects on the marine production due to a reduction in the whale population.

Research Aim

The primary aim of the present study is to assess and report the impacts of whale stock decline on the marine production.

Research Objectives

• To explore the impacts of whale stock reduction on marine food-web and ecosystem sustainability.
• To assess the role of whale population in enhancing marine productivity.

Chapter Two: Research Methods

Research Methodology

The present research adopted a systematic literature review methodology in order to identify, collect and critically appraise evidence from the previously published studies about the impacts of whale stocks decline on the marine production. With reference to the explanations by Alabool et al. (2018), Kraus, Breier and Dasí-Rodríguez (2020) and Pigott and Polanin (2020), systematic literature review is a detailed, critical and objective analysis which is focused on critically appraising the existing knowledge about the research phenomenon. Large number of previous studies on the impacts of whaling on marine ecosystem have adopted primary research methodologies. Therefore, the present study would focus on collecting relevant evidence from the previously published studies, critically review it and present resulting knowledge in a single document for easy accessibility and use during whaling and whale protection decision-making process. Even though Balduzzi, Rücker and Schwarzer (2019) and Xiao and Watson (2019) proposed meta-analysis as another key secondary methodology that may be used to critically appraise evidence from existing literature, the approach is only used when quantitative studies about the phenomenon of interest are to be appraised. Therefore, systematic literature review was appropriate for the present study as it widened the scope of the study by including both primary qualitative and quantitative studies for review hence leading to generation of more comprehensive information about the research phenomenon.

Literature Search Strategy

Quality of evidence collected for critical appraisal in different forms of literature reviews such as meta-analysis, scoping review and systematic literature review largely depends on the literature search strategy that was adopted (Jalali, S. and Wohlin, 2019; Page, 2018). Literature search strategy defines the keywords and search terms used for searching relevant literature, databases and the eligibility criteria to be applied during literature identification and selection (Lawrence, 2018; Younger and Boddy, 2019). Specific to the present study, literature search process was conducted on two major databases with high reputations of hosting top-quality and up-to-date literature on the fields of marine science and natural science. The databases include Biological Science Database and Environmental Science Database.

With reference to the explanations provided by Grewal, Kataria and Dhawan (2019) and Pearce and Chang (2018), the initial stage in literature search process involves identification of keywords to be used to select the most relevant literature for review. Furthermore, Bethard and Jurafsky (2018) and Timmins and McCabe (2019) recommended the use of Boolean operators such as “AND”, “OR” and “NOT” to combine different search terms and keywords keyed into the search boxes or lanes as an approach for restricting the search process to facilitate identification of literature which only contains the specified terms and keywords. Therefore, incorporation of Boolean operators into the literature search strategy helps in improving homogeneity of evidence collected for new knowledge synthesis. Selection and use of keywords and search terms in the present research was primary motivated guided by the explanations by Becker et al. (2021) that keywords should define the context of research phenomenon being explored, with the literature search process being comprehensively described in order to allow easy replication by future reviewers. During the literature search process in the present study, keywords employed include “whale” OR “baleen” OR “cetacean” OR “leviathan” OR “narwhal” OR “orca” OR “grampus” OR “beluga” OR “finback” AND “population” OR “stocks” OR “demography” AND “decrease” OR “decline” OR “reduction” AND “marine production” OR “marine ecosystem” OR “marine food-web”.

Eligibility Criteria

Inclusion Criteria

Inclusion criteria are standards which must be met by every literature in order to be selected for knowledge synthesis, and they are based on different literature characteristics such as the adopted methodology, research purpose, outcomes of interest and the involved population (Huls et al., 2018; Porzsolt et al., 2019). From the methodological perspective, only primary studies (both qualitative and quantitative studies) were included for review. According to Connelly (2020) and Patino and Ferreira (2018), systematic reviewers can ensure homogeneity of evidence collected from identified literature for review by only including studies with corresponding research purpose and outcomes. Therefore, only studies which assessed the impacts of whaling on marine productivity and ecosystem balance were selected for review in the present research. Furthermore, publication criteria such as date and language were applied during the literature search process, with only those originally published in English and in 2012-2022 were selected for review. Even though the use of translation services is recommended in the study by Swift and Wampold (2018), evidence from Porzsolt et al. (2019) shows that translation services may compromise the original quality of evidence as not every content would be perfectly converted to the preferred language.  Including studies published in 2012-2022 was key for ensuring collection of up-to-date evidence about the research phenomenon for new knowledge synthesis.

Exclusion Criteria

Exclusion criteria are standards used for discarding studies from the perspective of quality. Methodological approaches employed, research purpose and outcomes (Connelly, 2020; Huls et al., 2018). A study which fails to meet any of the inclusion criteria should be eliminated during the literature search process as a strategy for enhancing overall quality of reported findings. Specific to the present review, all the studies which assessed population decline of other marine organisms other than whales were excluded. Furthermore, studies which assessed other biological processes in whales apart from marine productivity were eliminated during the literature search process. According to Patino and Ferreira (2018) and Swift and Wampold (2018), quality and authenticity of evidence from secondary studies are not certain. Therefore, studies which involved collection and analysis of secondary data such as systematic reviews, meta-analysis, scoping reviews were eliminated. Full-text availability is another key exclusion criterion which was applied during the literature search process, with those studies with only abstracts available and grey literature being excluded. Summary of eligibility criteria which guided the literature search process is presented in the Table 1 below.

Table 1: Eligibility criteria used during the literature search process

Literature Search Results

Preliminary literature search on the two databases led to the identification of 79 records (38 records from Biological Science Database and 41 records from Environmental Science Database) which were further subjected to eligibility assessment in order to facilitate selection of most appropriate studies for the systematic literature review. In the duplicate removal stage, a total of 17 records were eliminated, with the remaining 62 records being subjected for additional assessment process based on the publication characteristics such as year and language. The process led to the elimination of additional 36 records: 13 published in non-English language and 23 published before 2012. Additional 14 papers were eliminated from the remaining 26 records: 5 papers without full-text format and 9 secondary research-based papers. The remaining 12 full-text papers were then taken though the manual quality assessment process where 7 studies were eliminated. The remaining 5 full-text research studies were finally selected and included in the systematic literature review process. Figure 1 below shows a PRISMA diagram describing the literature selection process.

Figure 1: PRISMA Diagram Showing the Literature Selection Process

Chapter Three: Results

Thematic analysis of the identified and selected 5 studies led to the identification of three themes describing the impacts of whaling on marine productivity. The themes developed include productivity enhancement, sequestering carbon, and nourishing the depths. In this chapter, evidence used for developing the three themes would be outlined and described.

Theme One: Productivity Enhancement

In addition to providing large amounts of meat, oil and blubber for human consumption, evidence from 3 out of 5 studies included in this systematic literature review shows that whales are also involved in the provision of significant marine ecosystem services (Fossi et al., 2014; Moore and Barlow, 2013; Tixier et al., 2020). Nonetheless, role of whales in marine and general ecosystem sustainability has been overlooked in most debates about whale conservation and commercial whaling (Fossi et al., 2014). According to Fossi et al. (2014), the faecal plumes of whales contain valuable nutrients such as nitrogen, iron and phosphorous which are involved in the stimulation of phytoplankton and microscopic marine algae production; all these microorganisms play an important role in forming the base for many marine food chains. From this perspective, reduction of whale stocks would interfere with the sustainably of marine food chains.  Additionally, a study by Tixier et al. (2020) established that the iron concentration in blue whale faeces (from Antarctica) is 15 million higher than that of the Antarctic seawater. Availability of irons influences growth of phytoplankton which is a key component of the marine ecosystem. Corresponding results were reported in the study by Moore and Barlow (2013) which noted that Southern Ocean-based blue whales through their faecal plumes are involved in increasing marine productivity by supporting the fisheries by approximately 240000 tonnes of organic carbon per year. Marine organisms require organic carbon for their survival.

Theme Two: Sequestering Carbon

Evidence about the role of whales in marine carbon regulation has been presented in all the five studies included in this review. Based on the fact that the phytoplankton use carbon dioxide to support their photosynthetic process, enhancing their productivity through the release of nutrients contained in the faecal plumes of whales would facilitate removal of such gases from the atmosphere (Fossi et al., 2014; Tixier et al., 2020). According to Moore and Barlow (2013), at least 15000 sperm whales from the Southern Ocean often deposit approximately 40 tonnes of iron into water surface every year, which helps in the phytoplankton productivity enhancement. Even though some concentrations of carbon from the phytoplankton are continually being recycled marine animals through feeding and defecation processes, at least 30-45% of such carbon often settle on the sea/ocean bottoms as phytoplankton decay hence locking up the carbon for a long period of time (Moore and Barlow, 2013). Consistently, Moore et al. (2019) reported that whales are involved in the sequestering of at least 200000 tonnes of carbon every year. On the other hand, evidence from the study by Frankel et al. (2021) shows that the sperm whales are often involved in the removal of at least 250000 tonnes of carbon from the atmosphere compared to the volumes they add during their metabolic activities such as respiration. Therefore, the continued decrease in the number of sperm whales in most of the marine ecosystems such as the Southern Ocean has negatively impacted effective regulation of carbon levels in those areas.

Theme Three: Nourishing the Depths

Three out of the five studies included in this review provided evidence showing the importance of whales in facilitating sustained fish and other marine organisms’ stocks. With reference to the evidence provided in the studies by Frankel et al. (2021) and Moore and Barlow (2013), whale falls facilitate the creation of habitat islands which benefit scavengers such as hagfish and sharks, shrimp, gastropods, bacteria in addition to a litany of other marine organisms. However, Moore et al. (2019) reported that the frequency of whale falls significantly decreased as a result of high levels of industrial whaling, a trend which consequently caused extinctions of significant number of anthropogenic species, especially those found in the deep sea. Even though not extensively justified, Moore et al. (2019) reported that biotechnological analyses have established that bacterial clones from the whale carcasses can be used for developing detergents for removing stains from the laundry during the cold-water washing process. Commercial adoption of such detergents would therefore lead to significant savings on energy, increased profits and improving cleaning process. Furthermore, Frankel et al. (2021) and Moore and Barlow (2013) reported that rich nutrients released by whales during the defecation process often play an important role in sustaining fish stocks, provision of food to other fish as well as enhancing realisation of a sustainable marine ecosystem. Therefore, all these benefits are likely to be missed if both commercial and industrial whaling processes are not properly regulated.

Chapter Four: Discussion

Impacts of Whale Stock Reduction on Marine Ecosystem Sustainability

The first objective of the present review was to explore the impacts of whale stock reduction on marine food-web and ecosystem sustainability. Specifically, this review has established that whales play an important role in enhancing marine ecosystem sustainability; hence reduction in their stocks would negatively impact the such sustainability. Specifically, evidence from the studies included in this review, Fossi et al. (2014), Moore and Barlow (2013) and Tixier et al. (2020), shows that the faecal plumes of whales comprises of valuable nutrients such as irons, phosphorous and nitrogen which are involved in the stimulation of phytoplankton production. Such microscopic marine algae are key components of the marine food chains. Therefore, whaling is likely to interfere with the sustainability of the marine food chain. Similar findings had been reported in the previous studies by Nøttestad, Fernö and Axelsen (2018) and Orams (2019) which established that both commercial whaling and human impacts have significantly influenced depletion of the marine species, particularly those located at the top level of the food web. From a logical perspective, removal of a predator from a food web, which in the marine ecosystem are whales, would influence a significant growth in the prey population.

However, the present review has established that most of the marine ecosystems are tremendously diverse and that the multifaceted inter-relationships between the predator-prey and species interactions are not comprehensively coupled because of different factors such as ontogenetic alternations in diet, switching of prey as well as cannibalism. Corresponding with the results from the previous studies by Lopez and Lopez (2017) and Saidala and Devarakonda (2017), the present review has therefore established that depletion in the number of large marine predators such as whales may lead to the development of significant and unforeseen problems in the entire ecosystem, impacting its sustainability. Therefore, it is justifiable to note that whales are not only involved in the provision of large volumes of blubber, meat and oil for human consumption but also being involved in enhancing sustainability of the marine ecosystem, despite their role in this context being overlooked in some of the debates regarding commercial whaling and whale conservations.

The present review, especially evidence from the studies by Moore and Barlow (2013) and Moore et al. (2019), has established that over whaling in both Antarctica and Southern Ocean has influenced the occurrence of long-term reduction in the general marine productivity, a decline which has consequently caused reduction in the krill population. Even though previous study by Kraus, Breier and Dasí-Rodríguez (2020) reported a significant increase in the number whale stocks within the Southern Ocean, this review has further established that the krill numbers have not successfully recovered to their original levels in the pre-industrial whaling periods. Specifically, such deficiencies are largely influenced by over-whaling and climate change in the marine ecosystem. Consistent with the results from the present review which showed that the blue whale faeces contribute to nitrogen supply in the Southern Ocean, previous study by Alamri, Alsariera and Zamli (2018) established that the marine mammals such as whales are involved in enhancing primary production within the marine feeding areas through consistent supply of nitrogen to the surface waters in the Gulf of Maine especially by releasing faecal plumes and urine. Therefore, a decline in the whale population is likely to compromise both sustainability and efficiency of the marine productivity.

Results from this review are further consistent with those reported in the study by Nøttestad, Fernö and Axelsen (2018) which established that endangered right whales from the Bay of Fundy played a primary role in enhancing primary productivity of marine ecosystem by producing both phosphorus and nitrogen in their faecal plumes. Evidence from the study by Frankel et al. (2021) further showed that the feeding behaviour of sperm whales from Hawaiian waters helped in transferring at least 150 tonnes of nitrogen to the surface waters from the deep-sea waters, hence influencing increased rates of marine production by at least 600 tonnes of organic carbon every tear. Consequently, significant decline in the number of sperm whales among other whale species in Hawaiian waters and Southern Ocean waters as a result of increased commercial whaling has made such marine ecosystems to lose large tonnes of new nitrogen annually hence interfering with the sustainability of the marine ecosystem.

The present review has acknowledged existence of ongoing debates regarding the role of large whales in the marine ecosystem and whether they should be considered as predators for fisheries resources, marine resources for harvesting or indispensable component of healthy marine ecosystems. Specifically, the studies by Frankel et al. (2021) and Tixier et al. (2020) established that whales play a central regulatory role in the marine ecosystems, contrary to the arguments presented in the studies by Alamri, Alsariera and Zamli (2018) and Mamzer (2021) that whales are key competitors of fisheries resources hence their numbers should be regulated in order to protect fish populations in the marine ecosystem. A key evidence developed from the present review is that whales have the ability of exerting strong pressures on the marine ecosystem either through direct predation or indirect food-web interactions. Specifically, Moore and Barlow (2013) outlined the significant role of whales as consumers in the marine ecosystem by reporting that at least 60% of the primary production at the North Pacific Ocean was required in order to sustain populations of large whales before the commercial whaling. As a result of the reduced whale stocks at the North Pacific Ocean, the productivity mechanism is currently shifted to other marine species and pathways within the marine food-webs (Moore et al., 2019). However, it is important to note that the levels of primary marine productivity might have been higher before the commercial whaling as the whales played a central role in inducing the recycling and retention of nutrients at the upper surface of the marine waters. 

The present review has established that the endotherm of total metabolic rate of whales is relatively high hence playing a key nourishing role in the marine ecosystem. On the contrary, evidence from the previous studies by Hoareau (2020) and Wangn et al. (2018) identified low rate of mass-specific metabolism compared to other smaller mammals in the marine environment as a key consequence of the immense size of whales. Evidence about the key feeding behaviours of whales has also been reported in three out of the five studies included in this review (Fossi et al., 2014; Frankel et al., 2021; Moore and Barlow, 2013). Specifically, Fossi et al. (2014) noted that food needed sustain a single blue whale can be used to support between 5 and 9 smaller minke whales or 1600 penguins. On the other hand, Mamzer (2021) argued that such smaller marine animals often have higher metabolic rates hence limiting their collective biomass to not more than 54% of the total biomass of the blue whale. As reported in the study Moore and Barlow (2013), a decline in the baleen whale populations has discriminatorily as consequently reduced the ability of marine ecosystems to retain the carbon levels at both the living biomass and carcases sinking to the bottom of the sea, even in situation where the primary productivity of the marine environment is held constant. Based on these findings, it is justifiable to note that whale stocks at large play a central role in regulating the marine carbon levels and that a reduction in their population is likely to interfere with the different metabolic activities within the marine environment.

In line with the arguments from the previous studies by Hoareau (2020) and Wangn et al. (2018), the present review has further identified whales are key predators in the marine environments which are involved in influencing the evolutionary and ecological dynamics of prey population, with the impacts being expressed on the food-webs as well as different biochemical cycles such as transportation of iron and nitrogen. For example, Early and Preble (2020) reported that the predation activities of baleen whales might have played a central role on the Calanus copepods’ evolutionary force in the Arctic, leading to the selection of shorter life spans, increased rates of growth and smaller sizes. With reduced population of baleen whales in the Arctic, evolutionary selection is currently favouring larger and longer-lived copepod species (Early and Preble, 2020). Correspondingly, this review has established that the near-extinction of whales in the Southern Ocean during the 20^th century might have influenced the release of other krill predators, including Antarctic fur seals and penguins from the competition.

Role of Whale Population in Enhancing Marine Productivity

The present study further assessed the role whales in enhancing marine productivity. In addition to being predators, whales are also prey in the marine food chain, and their availability plays a key role in promoting nutritional value of the marine ecosystem. With reference to the explanations by Frankel et al. (2021) and Tixier et al. (2020), large sizes of whales allow them to concentrate nutrients and energy in the marine environment, especially in areas where such resources are otherwise highly dispersed and limited in quantity. Contrary to most of the ecosystems where large body size is considered as a protective mechanism against predation, the present review has established that the marine realm is made up of predators with large body sizes and ability to subdue great whales, some of such animals include large raptorial cetaceans and shark with serrated teeth. As reported in the study by Moore and Barlow (2013), whales are essential prey for killer whales (Orcinus orca), decline in the population of great whales had very adverse indirect impacts on the marine productivity. Correspondingly, Saidala and Devarakonda (2017) established that at least 15% of the projected 60000 killer whales globally are exclusively feeding on the marine mammals. Therefore, general decline of the great whales has significantly led to the overall reduction in the killer whale stocks which acted as predators for the great whales or expanded their diets in order to include other prey species for their survival.

The reported findings in this review could be further justified using evidence from the previous studies by Baker and Clapham (2020) and Lopez and Lopez (2017) which demonstrated that killer whales in the NORTH Pacific Ocean purportedly started to extensively prey on smaller marine mammals such as Steller sea lions, harbour seals and sea otters because of the general reduction in the great whale populations. Reduction in the sea otters’ population has influenced release of herbivorous sea urchins, which is the preferred prey of otters, from the limitations associated with predation, hence leading to their elevated rates of herbivory in addition to a significant reduction in the densities of the coastal kelp forests. The effects of such reductions can be demonstrated using evidence from the previous studies by Nøttestad, Fernö and Axelsen (2018) and Orams (2019) which reported that the decline in coastal kelp forests within the Northern Pacific Ocean influenced a reduction in the primary coastal productivity, general population of fish in addition to a reduction in the marine sequestration of carbon. Therefore, it is justifiable to note that the relationships between killer whales and their prey, great whales, have significant impacts on the function and structure of the ocean ecosystem in both unexpected and diverse manner.

The present review has further established that whales acts as key vectors for both nutrients and material flux, which helps in enhancing marine productivity. Specifically, Fossi et al. (2014), Frankel et al. (2021) and Tixier et al. (2020) reported that whales enhance primary production of marine environment through different strategies such as horizontal transfer, vertical mixing and recycling of carbon and other limiting resources in the ocean. During their diving process, whales are able to facilitate production of the mechanical energy to the marine environment, a mixing effect which is key during the stratified conditions or in the presence of limited winds (Frankel et al., 2021; Moore et al., 2019). Evidence from the previous studies by Nøttestad, Fernö and Axelsen (2018) and Orams (2019) shows that nitrogen is a key limiting macronutrient in most of the marine ecosystems. Nonetheless, the present review has established that cetaceans among other marine mammals are involved in the delivery of large volumes of nitrogen to the photic zone during the process of feeding at or underneath the thermocline, leading to the excretion of metabolic faecal nitrogen and urea to the water surface. Therefore, the release of nitrogen through the faecal plumes helps in enhancing plankton productivity. During the process of feeding on deep-living prey as well as the defecation at the surface, sperm whales influence upward transportation of iron into the photic zone.

The present review has further demonstrated that whales often assimilate a relatively low volume of their dietary iron during the production of blubber and that their faecal plumes contain iron concentrations which are at least 15 million times higher than the ambient levels (Moore and Barlow, 2013; Moore et al., 2019). Therefore, the results from the present study can be used to explain the limited availability of iron at Southern Ocean which has experienced significant reduction in the sperm whale stocks due to commercial whaling. Contrary to the arguments by Alamri, Alsariera and Zamli (2018) and Hoareau (2020) that there would be a significant increase in the krill stocks following the decline in population of Southern Ocean whales, the present review has established that most of the areas within the ocean have demonstrated no significant increase, with higher chances of further decline in the whale stocks. The limited increase in whale stocks might have been influenced by the general decline in the whale-induced iron fertilisation which is required for the primary marine productivity (Early and Preble, 2020; Mamzer, 2021). Such trends are likely to interfere with successful recovery of some whale stocks through negative feedbacks. Furthermore, this review has established that great whales are involved in the enhancing horizontal transportation of limiting nutrients among other indispensable materials across different ocean areas; hence justifying the role played by whales enhancing marine nutrient cycling.

The present study has also identified whale falls as a key strategy for enhancing marine production. For example, Frankel et al. (2021) and Moore et al. (2019) noted that the whale carcasses are largely comprised of lipids and proteins, hence can lead to the yielding of large volumes of organic enrichment to realm which are nutritious and energy impoverished, when they settle to the seafloor. Similar evidence about high nutritious contents of whale carcasses has been presented in the previous study by Aoki et al. (2018) which reported that a single 40 tonne grey whale comprises of roughly 2 million grams of carbon, which is equivalent to at least 2000 years of background carbon flux. From an analogical perspective, whale falls have the ability of influencing deep-sea floor productivity as in the case of tree falls in the forest by promoting alteration of food availability, supporting varied biotic assemblages as well as provision of habitat structures. Comparative assessment of evidence in this review and the results from other previous studies provides a description of the significance of whale falls and carcasses in marine production at different stages. During the mobile-scavenger stage, soft tissues from whale carcasses are consumed hagfish, sharks among other necrophages (Frankel et al., 2021; Sayed et al., 2016). Thereafter, heterotrophic infauna is involved in the exploitation of the organically augmented sediments together with the lipid-enhanced whale bones at the enrichment-opportunist (Aoki et al., 2018; Tixier et al., 2020). During the last stage, sulphophilic phase, the sulphides which resulted from the anaerobic decomposition of the residual skeleton are involving in supporting free-living bacteria (Moore and Barlow, 2013; McInnes et al., 2020). The stage can last for at least three decades. Therefore, it can be noted that food-rich characteristics and prevalent incidences of whale falls have significant ecological and evolutionary impacts on the marine environment.

Chapter Five: Conclusion and Recommendations

Conclusion

The present study focused on achieving two main objectives, (1) to explore the impacts of whale stock reduction on marine food-web and ecosystem sustainability, and (2) to assess the role of whale population in enhancing marine productivity, which were both successfully met. Specific to the first objective, the present review has established that whales play an important role in enhancing sustainability of marine food-web and ecosystem, hence reduction in the whale stocks is likely to interfere with the normal functioning of the biological aspects of the marine environment. The present review has established that the faecal plumes of whales are made up of valuable nutrients such as nitrogen, iron and phosphorous which help in the stimulation of phytoplankton production. Specifically, the phytoplankton are found at the base of most of the marine food chains. Therefore, reduction or extinction of whale stocks would interfere with the phytoplankton production hence negatively impacting efficiency of most of the marine food chains.  

The present review has further established that whales play an important role in enhancing marine productivity, and that a decline in their stocks would negatively impact the marine ecosystem. Precisely, whales ensure marine productivity through four key strategies; as consumers in the food chain, as prey, whale falls and as vectors for transporting nutrients and materials. Whales have the ability of exerting strong pressures on marine communities through either direct predation or indirect interactions within the food-web. As preys, large sizes of whales allow them to concentrate nutrients and energy within the marine environments with highly dispersed or limited resources. Furthermore, newly evidence from this review shows that whales promote primary marine production by vertical mixing, recycling and horizontal transfer of carbon, nutrients among other limiting resources in the marine environment. The whale carcasses are made up of large volumes of lipids and proteins which when sink in the seafloor lead to the massive production of pulses of organic enrichments.

Recommendations for Practice

Reported evidence from the present review shows that extinction of whales or reduction in their stocks would have negative consequences on the marine ecosystem functions. Therefore, there is need to ensure proper implementation of IWC’s moratorium for whaling as a strategy for ensuring recovery and protection of whale populations. Even though some whale species such as the Southern right whales and North Pacific humpbacks are increasingly being recovered from the commercial whaling, there is need to put more emphasis on the other threatened whale populations such as Antarctic blue whales and North Atlantic right whales which have been reduced to a level they are currently experiencing the Allee effects. In addition to commercial and industrial whaling, human activities interfere with the natural marine ecosystem have also been identified as a key threat to whale population. Therefore, additional policies focusing on the marine environment protection and sustainability should be formulated and implemented in order to protect whale stocks and influence their recovery.

Recommendations for Future Research

The present study has reported the general roles played by whales in enhancing marine production. Therefore, future research in this context should focus on a specific whale species. Such studies would help in determining the key factors that have influenced significant reduction in the specific whale species stocks and facilitate formulation of policies which are tailored towards enhancing recovery of whale stocks for the defined species. Furthermore, additional research should be conducted in order to determine and report the possible impacts of the decline in populations for other marine predators (those preying on whales) such as shark on the overall whale stocks and marine production.

References

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Appendices

Appendix 1: Literature Matrix

Appendix 2: CASP Tool for Economic Evaluation