As a result of resource overlap, interactions between marine mammals and fisheries are a common occurrence (Trites et al., 1997; Liu et al., 2019). A wide spectrum of these interactions has been documented, from the mutually beneficial to the detrimental. Marine mammals have taken advantage of human activities, particularly, fishing, for the nutritional gain (Bearzi, 2002; Allen et al., 2014). In several parts of the world, “co-operative” or “associative” fishing between marine mammal species and fishing communities has led to mutually beneficial interactions, in that both the fishermen and the animals find easy target and forage fish, respectively (Northridge and Hofman, 1999; Neil, 2002; Peterson et al., 2008; Kumar et al., 2012; D’Lima et al., 2014). On the other end of the spectrum, interactions such as depredation and bycatch negatively affect and alter marine social-ecological systems (Tixier et al., 2021; Box 1). Depredation affects both marine mammal populations and fishery socioeconomics due to gear damage and catch loss, by creating artificial resource provisioning due to the introduction of novel prey resources, leading to higher chances of incidental entanglements (Werner et al., 2015). Bycatch is a major threat to the conservation and recovery of marine mammal population(s) worldwide (Avila et al., 2018) leading to the declines of a wide range of species such as dugongs (Dugong dugon) (Marsh et al., 2011), monk seals (Monachus monachus) (Woodley and Lavigne, 1991; Guüçluüsoy et al., 2004), and Hector’s dolphins (Cephalorhyncus hectori) (Slooten and Dawson, 2010), amongst others. The recent “human caused” extinction of the Baiji (Lipotes vexillifer) (Turvey et al., 2007), and the expected extinction of the vaquita (Phocaena sinus) (Rojas-Bracho et al., 2006; Jaramillo-Legorreta et al., 2007; Morzaria-Luna et al., 2012) have further added to the growing concerns of the impact of fisheries on marine mammals.

The large-scale capture of pantropical spotted (Stenella attenuata), spinner (S. longirostris) and common (Delphinus delphis) dolphins (Gosliner, 1999; Wade et al., 2007) in the Easter Tropical Pacific (ETP) tuna fishery was instrumental in the recognition of bycatch as a global threat to marine mammal populations and the subsequent development and implementation of the United States Marine Mammal Protection Act (MMPA) in 1972 (Gerrodette, 2009). Since then, large-scale commercial operations using gear such as gillnets, purse-seines, hook and line, and trawlers have been extensively reviewed to develop management strategies to reduce their impacts on marine mammals (Gilman et al., 2007; Tixier et al., 2021). Several aspects of these interactions, particularly, marine mammal entanglement in fisheries have been widely studied (Read et al., 2006; Moore et al., 2010; Reeves et al., 2013), especially: (i) well-documented bycatch in gillnets, trawls, longlines, purse-seines, pots and traps, etc.; (ii) poorly documented bycatch in artisanal or small-scale fisheries in developing countries, and (iii) the transition of incidental marine mammal catch in fishing gear from being discarded to gaining market value (Read, 2008). This increased understanding has provided the impetus for the development and implementation of marine mammal population recovery and maintenance measures (Perrin et al., 1994).

Despite this documentation, the management and mitigation of these interactions is a pressing concern (Anderson et al., 2020; Hines et al., 2020). The k-selected life-history traits of marine mammals: long-life spans coupled with relatively late sexual maturity and low reproductive rates, severely limit conservation efforts (Brown et al., 2014; Mannocci et al., 2014).

In their review of a decade of bycatch management in commercial fisheries, Dawson et al. (2013) categorized management measures into three main technical approaches to increase the understanding of their efficacy in attaining management goals: (i) strategies that change human behavior, either mandated or voluntary: e.g., spatial or place-based management, monetary incentives, etc.; (ii) strategies that change the nature of interactions: e.g., technological interventions like bycatch reduction devices in trawl fisheries, and gear modifications such as changes in hook design in longline fisheries; and (iii) strategies that change animal behavior: e.g., acoustic deterrent devices, acoustic harassment devices. These management strategies were developed and evaluated in the context of commercial fishery operations. The high economic and social costs of management, such as, heavy reliance on expensive technology, marginalization of fishing communities, inadequate governance and enforcement, limit their wider implementation in small-scale fisheries (Brotons et al., 2008; Mangel et al., 2013; Brownell et al., 2019) but these aspects were not explicitly considered in the Dawson et al. (2013) typology.

An alternative multidisciplinary framework for bycatch mitigation has recently been proposed by Squires et al. (2021). This “Bycatch mitigation hierarchy framework” offers a systematic order of actions to manage and mitigate bycatch that considers the human elements of bycatch reduction. This framework comprises four basic approaches: (1) private solutions, including voluntary, moral suasion, and intrinsic motivation; (2) direct or “command-and-control” regulation starting from the fishery management authority down to the vessel; (3) incentive- or market-based approach to alter producer and consumer behavior and decision-making; and (4) a hybrid of direct and incentive-based regulation through liability laws.

Managing marine mammal interactions with commercial and small-scale fisheries requires deep understanding of their social, economic, and cultural linkages to fishers’ livelihoods (Carvalho et al., 2011). However, the current definitions of fishery operations are relatively simplistic in the context of management (Box 2). For small-scale fisheries in particular, these definitions are nuanced, and vary spatially (Teh and Pauly, 2018). Therefore, finding an appropriate balance between the ecological and human dimensions of marine mammal interactions with fisheries is a major challenge (D’Lima et al., 2014).

Despite the acknowledgment of these challenges, the long-term efficacy of management measures implemented so far, is seldom considered by stakeholders in the overall conservation management process. This ambiguity leads to a general lack of focus and consensus on the critical role of place-based management measures to mitigate these interactions. The overall aim of this review, therefore, is to understand the scope of research on marine mammal interactions with fisheries and discuss the implications of the management measures aimed at reducing the impacts of these interactions, particularly in the context of small-scale fisheries.

We used two bibliographic databases, SCOPUS, and Web of Science to identify a range of peer-reviewed articles, published and unpublished reports, and conference papers related to primary research on marine mammal interactions with fisheries. We did not use Google Scholar as it is not recommended for systematic or scoping reviews due to its unreliability in identifying articles specific to the keywords (Haddaway et al., 2015).

After initial deletions based on duplicates and relevance to the search terms, the list was reduced to 784 articles. Of these, 489 articles were excluded by screening the titles and abstracts for relevance to key words (Supplementary Material). Further stringent inclusion and exclusion criteria were applied to the remaining articles, based on the overall aims of the review, i.e., to understand the overall research trends in marine mammal interactions with fisheries. Review articles, fisheries management per se, marine mammal behavior and ecology studies, that were not directly related to interactions, were excluded from the review.

Relevant additional articles and gray literature (e.g., reports, conference, and meeting proceedings, etc.) were mined through the available reference lists of articles and were analyzed based on the above criteria. Limited or poor bibliographic information, however, is a major challenge in the accessibility of gray literature. Several reports and proceedings were inaccessible using search engines like Google Scholar, Elsevier, Scopus, etc., or through university libraries.

Thematic analyses were conducted on the final list of manuscripts (271 articles: published articles 263, gray literature: 8 documents) to understand the general trends in research, based on the year of publication, region where research was conducted, type of fishery operation (commercial or small-scale fishery, Box 1), and taxonomic group (cetacean, pinniped, sirenian, etc.). The literature was then segregated into two main emergent themes related to the interactions between marine mammals and fisheries (Box 2): (A) Direct interactions: mainly, (i) bycatch, and (ii) depredation, and (B) Indirect interactions: (i) trophic interactions, to compare trends over time across fishery operations and, to understand which of these interactions are the most reported in the literature.

Further, management strategies and the challenges faced to mitigate marine mammal interactions with fisheries were collated and discussed in accordance with the technical categories described by Dawson et al. (2013) (see Introduction) because we considered these categories appropriate for the current literature.

There are two limitations to our study: (1) the literature reviewed is a non-random sample from the available literature driven by the key-words we used in our search; (2) our analyses are based on the results described in this literature and are not a statistical comparison between experimental or control studies.

We first document the global trends in research on marine mammal interactions across commercial and small-scale fisheries, and various fishing gears. We then present the thematic analyses of research on direct and indirect interactions, followed by a discussion of the observed management responses and challenges across fishery operations and interactions.

Based on the intensity of interactions, or their effects on marine mammal populations or fishery operations, management approaches aim to achieve set goals or conservation priorities. The success or failure of these goals can be recognized by certain indicators, for example, increased recruitment in and/or mortality reduction of the target population(s), and improved usage of the area by stakeholders, etc. (Hoyt, 2011; di Sciara et al., 2016). All efficacy metrics are dependent on prior information about the system under consideration, such as: (i) baseline population or abundance estimates, (ii) mortality records of the target species, (iii) stakeholder usage of the area, and (iv) the scale of the fishery operation. However, there are inevitable uncertainties regarding data on marine mammal population(s), fishery socioeconomics, and both human and animal behavioral ecology that limit the assessment of management efficacy in several ways (Table 2).

First, evaluating the success of place-based conservation measures and technological interventions is challenging because of the uncertainties associated with prior information on marine mammal abundance and identification of strategic habitats. In many instances, information on marine mammal abundance is inadequate, and data on most species are restricted to stranding records or from fisheries landing centers (IUCN, 2019). For example, for coastal species like the Indian Ocean humpback dolphin (Sousa plumbea), which is widely distributed around the peninsular Indian coastline, population estimates are fragmented, and most of the information on bycatch comes from incidental strandings and is thus mainly anecdotal (Sutaria et al., 2015; Braulik et al., 2017).

Second, robust and long-term population estimates require significant financial investment over extended periods. Such information is difficult to obtain, especially in developing countries (Moore et al., 2010; Lewison et al., 2014). In these scenarios without prior information, designing place-based measures poses a challenge to setting realistic goals and evaluating their success in the long run (di Sciara et al., 2016).

Third, bycatch projections for commercial fishery operations are based on existing bycatch estimates which are derived from fishery-observer surveys, and compiling and collating logbook data (Julian and Beeson, 1998; Morizur et al., 1999; Norman, 2000; Majluf et al., 2002; Underwood et al., 2008; Kindt-Larsen et al., 2016). While observer surveys have been an invaluable source of data (Edwards and Perrin, 1993; Morgan et al., 2002), accurately projecting these estimates over entire fishing fleets is not a straightforward process since observer coverage is often inadequate and the presence of an observer can influence fishing practices (Curtis and Carretta, 2020).

Small-scale fisheries present additional challenges with respect to bycatch monitoring, because of their spatial and operational variability, and unregulated, unstructured working environment (Hines et al., 2020). Methods such as rapid survey interviews, spatial risk assessments and monitoring catch at landing centers have provided baseline bycatch estimates for small-scale fisheries in developing countries (Pilcher et al., 2017; Temple et al., 2019; Hines et al., 2020; Verutes et al., 2020). However, fragmented monitoring efforts in small scale fisheries and the resultant inconsistencies in or the lack of bycatch data hamper the successful implementation of mitigation measures in small-scale fisheries (Gilman et al., 2010; Teh et al., 2015).

Data on marine mammal space use and behavioral observations, in the context of fishery operations, are being used for spatial risk assessments, understanding the drivers of bycatch, and assessing the effectiveness of implemented management measures (Grech et al., 2008; Marsh et al., 2011; Cerutti-Pereyra et al., 2020). Studies on acoustic activity and behavior of marine mammals have also assisted in understanding the precursors and the intensities of these interactions (Iriarte and Marmontel, 2013; Lewison et al., 2014; Kindt-Larsen et al., 2016; Lopes et al., 2016; Clay et al., 2019). Over the past decade, researchers have stressed the importance of ecological studies to understand the underlying causes of bycatch (Northridge et al., 2017).

Marine debris and the transition of bycatch from discard to commodity are two emerging concerns documented in the available literature. Since the mid to late-2010, data from stranding records and carcass examinations has been used to augment bycatch estimates to evaluate the risks due to all anthropogenic activities, mainly fisheries, and to understand the fate of bycaught animals. Analyses of injuries on carcasses have revealed significant number of entanglements of marine mammals in marine debris from small-scale fisheries, indicating interactions with fishing gear or ghost nets (Kaiser et al., 1996; Franco-Trecu et al., 2017).

The transition of bycatch from discard to commodity is also a growing concern in the light of fishery resource depletions (Ermolin and Svolkinas, 2018). Incidental catches create opportunities for food procurement, as declining fish catches widen the gap between supply and demand for food and income, particularly in developing countries (Robards and Reeves, 2011; Leeney et al., 2015; de Boer et al., 2016). Recent studies have highlighted the urgency of the issue. Marine mammals have been targeted for bait (Campbell et al., 2020; Briceno et al., 2021) or to attract fish to modified fish aggregating devices (Castro et al., 2020).

This process increases the likelihood of a fishery transitioning from bycatch to unregulated and directed harvest of marine mammals (DeMaster et al., 2001), exacerbating the threats to marine mammal populations. These factors should also be included in existing management strategies.

Human behavior is another major challenge to the uptake and application of management. Some measures are unlikely to be adopted, unless mandated and enforced. Even with mandates and regulations in place, the compliance levels depend upon the local social and economic conditions (Whitty, 2018). For example, fishing restrictions, quotas, or blanket bans on fisheries, can adversely affect community livelihoods and introduce human-human conflict within social-ecological systems, making management of interactions even more complicated (Dickman, 2010). Furthermore, in areas where marine mammals are protected by law, fishers are often unwilling to report interactions altogether for fear of persecution (Torres et al., 2018). For interactions such as marine mammal depredation of fisheries, the observed disparity between reported and actual depredation levels, particularly for small-scale fisheries, is high (Bearzi et al., 2011). Therefore, although the frequency of depredation may be lower than reported, the perceived economic damages due to depredation are likely to be higher than they actually are (Sepulveda et al., 2018). In certain cases, there may not be a link between depredation events and the involvement of marine mammals (Bearzi et al., 2008). Several other factors may cause gear damage and catch loss, for example, fish or other invertebrate species, or marine debris (Lauriano et al., 2009), which need to be considered in management strategies.

Incentive-based management measures, such as alternative livelihoods have been advocated as a practical and successful measure to manage interactions. However, such approaches may have limited success in poverty alleviation or be unsuitable for certain areas, particularly remote areas in low income countries, and for small-scale fisheries (Marsh et al., 2011; Squires et al., 2021). If implemented at all, such activities need to be carried out in accordance with proper guidelines (Armitage et al., 2009) and regulations, if unintended consequences to local communities, are to be avoided.

The wider implementation and effective enforcement of conservation measures can be severely limited by social-economic factors (Mendoza-Portillo et al., 2020). For example, small-scale fishers in the developing world are unlikely to have the financial resources to implement technological interventions. Even with subsidies to purchase these devices and adequate participant training and awareness, their voluntary long-term usage and maintenance and ultimately, their efficacy is highly unlikely (Brotons et al., 2008). For example, the vaquita population is dangerously low, despite the considerable investments in the gillnet ban by the Mexican government. This population decline is largely fueled by the high demand for and high market value of the totoaba (Totoaba macdonaldi) coupled with corruption (Brownell et al., 2019).

We were concerned to find that, despite the key role of human behavior and socioeconomics in the management context, only eight of the articles we considered propose the incorporation of human dimensions, mainly, socioeconomics of fisheries, cultural significance of livelihoods, and stakeholder belief systems (Szteren and Lezama, 2006; Bearzi et al., 2011; Morteo et al., 2012; Iriarte and Marmontel, 2013; Cook et al., 2015; Leeney et al., 2015; Snape et al., 2018; Mustika et al., 2021).

In addition to human behavior, the behavioral ecology and life-history traits of marine mammals are also major challenges to evaluating the long-term success of management measures. Marine mammal behavior plays an important role in the design and testing of technological interventions. For example, acoustic deterrent devices, or pingers have been shown to be unsuccessful in the long term in deterring marine mammals that exhibit behavioral plasticity. Especially for species with high site fidelity, pingers usually cause “habituation” or act as a “dinner bell,” facilitating higher interaction rates (Brownell et al., 2019). For instance, pingers successfully reduced Franciscana bycatch in bottom-set gillnet fisheries, and common dolphins and beaked whale bycatch in drift gillnet operations. However, Cox et al. (2004) concluded that after the initial success in bottlenose dolphin bycatch reduction, these devices were unlikely to further reduce their bycatch rates in gillnets.

Integrating data on marine mammal life-history traits is also vital to setting pragmatic goals for the management of interactions, particularly for bycatch mitigation. Fisheries bycatch models generally assume relatively a large population sizes of the by-caught species and seldom consider the impacts of the Allee effect (Wade and Slooten, 2020), i.e., reduced population growth rate at small population sizes. The Allee effect can have a significant influence on small, restricted populations, for example, coastal small cetaceans (Allee et al., 1949; Berec et al., 2007). Furthermore, the k-selected life-history traits of marine mammals, i.e., low reproductive rate and relatively slow rate of population growth, further limit population recovery (Mannocci et al., 2014). In such scenarios, even if fisheries-induced risks remain constant, bycatch could be a rare event with very deleterious population impacts, especially on small declining populations (di Sciara et al., 2016).

To effectively manage the resource overlap between marine mammals and fisheries, management guidelines should include social and economic dimensions of fisheries and marine mammal ecology. These measures could help in setting pragmatic and achievable conservation priorities based on enhanced understanding of critical knowledge gaps. The heterogeneity in the cultural, social and political nature of small-scale fisheries warrants a holistic understanding of these knowledge systems to implement temporal and spatial management measures. Bycatch is a major conservation concern in marine ecosystems, that requires a multidisciplinary approach to mitigation (Lent and Squires, 2017; Squires et al., 2021).

In addition to the occurrence, abundance and behavior of marine mammals, information on the effects of artificial resource provisioning on predators can shed light on how trophic interactions affect other functional groups in the ecosystem, including prey species (mainly fish stocks), and therefore fisheries (Tixier et al., 2019). However, there is also a need to develop social and economic perspectives of these interactions for the application of any mitigation measures to be contextually successful, both temporally and spatially. An area-specific adaptive management framework could therefore be an effective tool in reducing the risk to marine mammals from fisheries by coupling technical solutions with socio-economic and political interventions.

Marine mammal interactions with fisheries occur frequently. Marine mammal bycatch in fisheries is widely studied in literature and remains a major conservation concern due to its detrimental effects on marine mammal populations. However, despite its occurrence in many fishery operations, the effects of depredation on marine mammal ecology and fishery operations have rarely been quantified (Mooney et al., 2009). Depredation poses short-term benefits for marine mammals, creating new foraging opportunities directly facilitated by fishing operations (Tixier et al., 2015, 2020; Esteban et al., 2016). Such interactions could also result in increased chances of entanglement, injuries, and bycatch (Lewison et al., 2014; Guinet et al., 2015; Werner et al., 2015) due to the proximity of marine mammals to active fishing gear.

The biggest management challenge is our inadequate understanding of the social dimensions of these interactions and the inevitable uncertainties concerning both animal and human behaviors (Panagopoulou et al., 2017; Whitty, 2018; Mendoza-Portillo et al., 2020). Human behavior and fishery socioeconomics, in particular, play a key role in effective management of interactions and very few studies address these limitations (Northridge et al., 2017; Lewison et al., 2018).

An adaptive management response, i.e., (i) identifying management challenges and (ii) addressing the related data gaps in both the ecological and human dimensions should be used to set context-specific research and management priorities. Such a multidisciplinary approach could help in underlining the best possible measures for mitigating bycatch based on the local social-ecological conditions (Lent and Squires, 2017). For example, for a given region, with prior information on the fishery operations, and available data on marine mammal interactions with fisheries, the challenges could be evaluated against any existing management actions. This exercise could be then used to identify area-specific research gaps and to set pragmatic targets for research, management and social well-being in that area.

Clear conservation policies coupled with political will and capacity are important for the meaningful implementation of mitigation measures (Kuiper et al., 2018). With the limitations of technological solutions, coupled with socio-economic and political challenges, an adaptive management framework can be an effective tool in reducing the risk to marine mammals from fisheries. Nonetheless, we conclude that despite the vast body of literature on this subject, a “silver bullet” management solution to marine mammal interactions with fisheries does not yet exist (Bearzi et al., 2011; Snape et al., 2018).

KJ, HM, and DS contributed to the conception of the study. KJ conducted the review, analyzed the data, and wrote the first draft of the manuscript. All authors contributed to the editorial and review process and approved the final version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.