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Incidental mortality of dolphins in the eastern Pacific Ocean purse-seine fishery: Correlates and their spatial association
Abstract
A zero-inflated Poisson model was used to identify typical fishing practices that contributed to incidental mortality of dolphins in the eastern Pacific purse-seine fishery between 1993 and 2001. The presence of hazardous net conditions (net canopies and net collapses), the duration of the backdown procedure (the primary method of releasing dolphins from the net), the size and species composition of the encircled dolphin herd and the amount of tuna encircled, were all found to consistently contribute to increased dolphin mortality per set. In particular, the presence of net canopies and large biomass in the net contributed to both the development of problematic situations in which mortality could occur and to the mean mortality per set, once a problematic situation had developed. On the other hand, lengthy backdown procedures and the presence of net collapses contributed to the development of problematic situations, but had less effect on the mean mortality per set once a problematic situation had developed. Because some of these variables are partially correlated, the overall conclusion of this analysis is that one of the primary causes of dolphin mortality continues to be the encirclement of large herds. Dolphin mortality can increase with the number of dolphins encircled because: (1) the more animals encircled, the greater the likelihood of entanglement and mortality while confined in the net; and (2) the duration of the backdown procedure increases with the number of animals encircled. The duration of the backdown procedure may, in turn, contribute to increased dolphin mortality by: (1) keeping dolphins in close contact with the net for longer periods of time, thereby increasing the chances for entanglement; and (2) leading to the formation of net canopies. Dolphin mortality increases in the presence of net canopies because animals can be trapped below the sea surface in the areas of canopies. Spatial distributions of encircled herd size, duration of the backdown procedure, presence of net canopies and presence of dolphin mortality show similar patterns. Encircled herd size tended to be greatest south of the equator and north of the equator along the offshore margin of the fishery. In these areas, the duration of the backdown procedure tended to be longer and there was often an increased probability of net canopies and dolphin mortality, but also larger catches of tuna. These consistent spatial patterns suggest that reallocation of fishing effort to other areas may be an effective means of reducing the current level of dolphin mortality. Predictive models could be developed to assess tradeoffs between dolphin mortality and tuna catches at varying levels of fishing effort in areas where large herds are targeted by fishermen and different strategies for reallocation of fishing effort to other areas or to purse-seine sets on unassociated tunas.
... Dolphins that do not escape from the net during backdown are assisted over the corkline by vessel crew working from a raft within the pursed net. Dolphins can also become entangled in billows of netting ("net canopies") or in areas where sides of the net have come into contact ("net collapses"), contributing to mortality (Lennert-Cody et al., 2004). Divers work from within the net to release these entangled dolphins below the surface. ...
... https://www.iattc.org/PDFFiles/AIDCP/_English/AIDCP_Maintaining% 20qualified%20captain%20list.pdf 3 Eastern spinner and common dolphins have higher mortality rates than offshore spotted dolphins(Lennert-Cody et al., 2004), related to behavior within the pursed net(Pryor and Norris, 1978). 4 http://www.iattc.org/PDFFiles/AIDCP/_English/AIDCP.pdf ...
Multispecies aggregations of tuna, dolphins, and seabirds are prevalent and conspicuous in the vast waters of the eastern tropical Pacific and form the basis of a commercial fishery for yellowfin tuna (Thunnus albacares) through setting on schools of dolphins, which is among the largest tuna fisheries in the world. Incidental dolphin mortality associated with the development and early years of the fishery was high; by 1993 it was estimated that eastern spinner dolphins (Stenella longirostris orientalis) had been reduced to 44% and northeastern offshore spotted dolphins (S. attenuata attenuata) to 19% of pre-fishery levels. Efforts to reduce this mortality began at the inception of the fishery and comprised a diverse array of approaches: modifications to fishing methods and fishing gear (backdown, Medina panel, high-intensity floodlights, swimmers to disentangle and release dolphins); U.S. legislation (through the U.S. Marine Mammal Protection Act, MMPA, and subsequent amendments); international agreements (including the International Dolphin Conservation Program that established dolphin mortality limits, and the legally binding multilateral Agreement on the International Dolphin Conservation Program); and economic incentives [notably through establishment of the U.S. dolphin-safe label and positive certification by the Marine Stewardship Council (MSC)]. Together, these bycatch mitigation efforts have been remarkably successful; dolphin mortality due to entanglement as recorded by fisheries observers (hereafter, entanglement mortality) has been reduced by > 99%. Despite this, the degree to which dolphin populations have recovered remains unclear. Multiple lines of evidence indicate that individual dolphins experience multiple sets in their lifetimes and although causality has not been established, research suggests that chase and encirclement might have impacts on dolphins in addition to entanglement mortality. These impacts potentially include increased fetal and/or calf mortality, separation of nursing females and their calves, decreased fecundity, increased predation, disruption of mating and other social systems, and ecological disruption. The strong management emphasis on monitoring entanglement mortality, and the infrastructure necessary to support this monitoring (in particular, 100% observer coverage on large purse-seiners) require funding to the extent that other activities, particularly continued surveys to monitor stock status and clarify the potential influence of other effects of the fishery on dolphin populations, are currently inadequately funded.
... Millions of dolphins were killed from the 1960s until the 1990s due to the tuna-dolphin associations and the intentional encirclement of some species of dolphins to capture the tuna. Since then regulation, education and modification of fishing gear and practices contributed to reduce the mortality by over 95% (Hall and Boyer, 1986;Lennert-Cody et al., 2004;Gjertsen et al., 2010). Studies on cetacean interactions with purse-seine fisheries targeting small pelagics are scarce. ...
... The Portuguese continental coast is an area of known upwelling regimes near the coast, which cause significant variation in primary productivity, thus leading to variation and patchiness in the occurrence of schools of small pelagic fish (Borges et al., 2003;Silva et al., 2009), which are targeted by common dolphins as one of their primary prey (Silva, 1999;AM, unpublished data). Cetaceans are frequently caught worldwide in purse-seine fisheries, although most related studies have been directed towards the purse-seine fisheries targeting tuna in the Pacific (Hall and Boyer, 1986;Lennert-Cody et al., 2004) and only a few exist regarding purse-seine fisheries targeting small pelagics (e.g., Hamer et al., 2008). Moreover, few studies include sufficient data to estimate capture rates for an entire fishery or region. ...
Interactions between cetaceans and the purse-seine fishery operating along the whole Portuguese continental coast were studied based on on-board observations from 2010 to 2011. Cetacean presence and mortality were estimated and characteristics under which interactions were most likely to occur were identified. Observations were made on 163 fishing trips (0.7% of the average annual number of fishing trips) and 302 fishing operations/hauls. Cetaceans were present during 16.9% of fishing events; common dolphins (Delphinus delphis) accounted for 96% of
occurrences, mostly overnight in summerand early autumn. Regression models showed that cetacean presence during a fishing setwas significantly (p , 0.05) associated with sardine catches, effort, and latitude/longitude. Encirclement and mortality occurred in 2.3 and 1.0% of fishing events, respectively. Encircled species were thecommondolphin, bottlenose dolphin (Tursiops truncatus), and harbour porpoise (Phocoena phocoena), but only common dolphin showed mortality (three individuals); raised to fleet level, estimated total mortality rates of common dolphinswere 69 (95% CI 37–110) in the north and 91 (95% CI 55–165) in the south for 2010 and 78 (95% CI 47–140) in the south only for 2011. The estimated annual
mortality rate due to purse seining is 113 (95% CI 3–264)commondolphins, which is�0.63% of the current most optimistic estimate of population size for the Portuguese fishing area (SCANS II). The wide confidence limits, as well as variation between years, reflect low observer coverage, emphasizing the need for increased monitoring to cover gaps in the spatial and seasonal distribution of observer effort and provide reliable estimates of bycatch.
... For example, shark discards relative to tuna landings have been estimated to be 60 -90% for some longline pelagic fisheries (Harrington et al., 2005). Nevertheless, bycatch in tuna purse-seines is comprised of a large variety of species, including sea turtles, rays, sharks, and marine mammals, which indicates the need for careful monitoring (Romanov, 2002;Lennert-Cody et al., 2004;Amandè et al., 2010;Gilman, 2011). The magnitude and species composition of bycatch depend on the fishing practice used by tuna purse-seiners. ...
... Sampling coverage during the study period was about 4.6% of trips, i.e. less than half of the target rate of 10%. This coverage level is very low compared to the equivalent observer programmes in the eastern and western parts of the Pacific Ocean, which have been reaching almost 100 and 20% coverage, respectively (Hall, 1999;Lennert-Cody et al., 2004). Consequently, almost all bycatch species are even more poorly estimated than expected. ...
Amandè, M. J., Chassot, E., Chavance, P., Murua, H., Delgado de Molina, A., and Bez, N. 2012. Precision in bycatch estimates: the case of tuna purse-seine fisheries in the Indian Ocean. – ICES Journal of Marine Science, 69: .
Estimating bycatch, i.e. the incidental catch of non-target marine animals and undersized individuals of target species, by raising observer data to the whole fishery is routine practice. The annual bycatch of the European tropical tuna purse-seine fishery over the period 2003–2009 was estimated at 11 590 t [95% confidence interval: (8165–15 818 t)], corresponding to 4.7% of the tuna landings. An analysis of the variability in the precision of this estimate, based on generalized linear models and Monte Carlo simulations, showed that the current sampling coverage of the tropical tuna fishery observer programme, which is 4.6% of the fishing trips, resulted in large uncertainties in bycatch estimates by species, i.e. none of the estimates have a relative root mean square error smaller than 50%. Although the overall magnitude of bycatch of the fishery appeared to be small, the current sampling coverage was insufficient to give any reliable estimate for low-occurring species, such as marine turtles, some oceanic pelagic sharks, and some billfishes. Increasing the sampling coverage would likely improve bycatch estimates. Simulation outputs were produced to help define (i) trade-offs between the priority species to be monitored, (ii) the estimation precision, (iii) expected accuracy, and (iv) the associated sampling costs.
... Millions of dolphins were killed from the 1960s until the 1990s due to the tuna-dolphin associations and the intentional encirclement of some species of dolphins to capture the tuna. Since then regulation, education and modification of fishing gear and practices contributed to reduce the mortality by over 95% (Hall and Boyer, 1986;Lennert-Cody et al., 2004;Gjertsen et al., 2010). Studies on cetacean interactions with purse-seine fisheries targeting small pelagics are scarce. ...
... The Portuguese continental coast is an area of known upwelling regimes near the coast, which cause significant variation in primary productivity, thus leading to variation and patchiness in the occurrence of schools of small pelagic fish (Borges et al., 2003;Silva et al., 2009), which are targeted by common dolphins as one of their primary prey (Silva, 1999;AM, unpublished data). Cetaceans are frequently caught worldwide in purse-seine fisheries, although most related studies have been directed towards the purse-seine fisheries targeting tuna in the Pacific (Hall and Boyer, 1986;Lennert-Cody et al., 2004) and only a few exist regarding purse-seine fisheries targeting small pelagics (e.g., Hamer et al., 2008). Moreover, few studies include sufficient data to estimate capture rates for an entire fishery or region. ...
... Some communities of bottlenose dolphins even specialize in this type of foraging (Chilvers & Corkeron, 2001;Chilvers et al., 2003;Jaiteh et al., 2013). Similarly, in some areas, common dolphins (Delphinus delphis) have shown close association with fishing vessels, with bigger groups found in the presence of purse seiners and other fishing vessels (Lennert-Cody et al., 2004;Hamer et al., 2008;de Boer et al., 2012). The attraction response behaviours of both bottlenose and common dolphins towards fishing vessels contrast with avoidance responses often displayed towards other vessel types documented in the study area (Baş et al., 2017b). ...
Marine traffic has both short- and long-term effects on cetacean behaviour, yet fishing vessels present a unique situation as they disturb cetaceans whilst potentially offering alternative foraging opportunities. The Istanbul Strait is a key area for the study of anthropogenic disturbance on cetaceans due to heavy human pressure in a narrow space where at least three cetacean species are regularly encountered. The present study investigated changes in behaviour of bottlenose dolphins, common dolphins and harbour porpoises in relation to the presence of purse seiners using Markov chains analysis. The results revealed increased foraging in bottlenose and common dolphins’ behavioural budgets and a decrease in the time spent foraging by harbour porpoises. Moreover, a loglinear model was used to analyse the effect of possible confounding variables on observed behaviours. The only variables found to be affecting behaviour were the previous recorded behaviour, seiner presence and marine traffic density. Consequently, the presence of purse seine vessels leads bottlenose and common dolphins to change their behaviour and are related with a decrease of energy intake in porpoises. The results of this study reveal that there is an effect of purse seine vessel presence on the cetaceans found in the Istanbul Strait with potentially significant impacts on their behaviour, therefore we suggest more research is needed in the area to identify the long-term impacts of these observed behavioural changes.
... For instance, in the case of whales, the required observer cov- erage is 100% for the Atlantic shark gillnet fishery, during those times of the year when whales are calving (NMFS, 2002). In the Pacific Ocean, purse-seine observer programs have covered 20- 100% of the fishing effort ( Lennert-Cody et al., 2004;Hall and Roman, 2013). The coverage levels of at least 50% of total effort for rare species would give reasonably good estimates of total bycatch of rare species (see review in Babcock et al., 2003). ...
This thesis is a comprehensive work aiming to improve scientific knowledge on the green turtle (Chelonia mydas) in order to provide key scientific evidences needed for the implementation of coherent and effective management measures to protect at the Western Indian Ocean scale this threatened species. In a first step, this work aimed to established baseline data on the abundance of green turtles nesting females and long term trends of some key nesting populations of the region by applying different modelling methods. In a second step, this work determined the regional genetic structure of this species and the relationships that exists between the different populations. Finally, the conservation of marine turtles being closely dependant to external pressures, this work tried to characterize theanthropogenic pressures they face, more specifically those related to fishing activities. All these results allowed unraveling some key gaps on the biology and ecology of the green turtle in the region and led to a global vision of the conservation status of this species in the Western Indian Ocean. The compilation of the results enabled the identification of regional priority areas for protection, but also some more specific threatened sites such as Europa. Finally, this synthesis shedslight on research priorities and scientific approaches to be promote in the future to unlock other keyscientific issues and refine conservation priorities, not only of marine turtles, but also of marine megafauna as a whole.
... A common management approach for ensuring population viability in cetacean species is to define a tolerable threshold of incidental removal, termed potential biological removal (PBR) (Wade 1998, Slooten et al. 2006, to mitigate or minimize anthropogenic threats. In practice, the causes of incidental mortality are often generalized to fishery bycatch (Caswell et al. 1998, Read & Wade 2000, Dans et al. 2003, Lennert-Cody et al. 2004, Moore & Read 2008. The PBR calculation for cetaceans is based on an intrinsic population growth rate that is usually set as 0.04 when demographic data are not available (Wade 1998). ...
Predictive modeling of population trends can indicate the rate of population decline and risk of extinction, providing quantitative means of assessing conservation status and threats. Our study tests the rate of population change and risk of extinction of the Indo-Pacific humpback dolphin Sousa chinensis off the west coast of Taiwan, the only humpback dolphin population classified as Critically Endangered (CR) by the IUCN Red List of Threatened Species. Under the most optimistic assumptions, almost 60% of simulations (out of 250 replications × 5000 iterations) predicted that population decline will exceed 80% within 3 generations, while the mean estimate of population decline within 1 generation was >50% of the current population numbers. Status classification performed using IUCN Red List Categories and Criteria Version 3.1 supported previous CR classification, while risk assessment models that factored in anthropogenic impacts further increased the estimated extinction risk. At an adult survival rate of 0.95, a modeled increment of annual bycatch rate by 1% of population size increased the probability of extinction within 100 yr by 7.5%; this increase was lower at a higher adult survival rate. The estimated extinction risk was greatest under the impact of habitat loss, reaching a hazardous level when habitat carrying capacity dropped to less than 50%, indicating that habitat fragmentation and alteration of coastal environments pose the greatest threats to this population, even if the cumulative sum of fragmented patches of habitat may superficially appear to be large.
... For instance, in the case of whales, the required observer coverage is 100% for the Atlantic shark gillnet fishery, during those times of the year when whales are calving (NMFS, 2002). In the Pacific Ocean, purse-seine observer programs have covered 20– 100% of the fishing effort (Lennert-Cody et al., 2004; Hall and Roman, 2013). The coverage levels of at least 50% of total effort for rare species would give reasonably good estimates of total bycatch of rare species (see review in Babcock et al., 2003). ...
... A common management approach for ensuring population viability in cetacean species is to define a tolerable threshold of incidental removal, termed potential biological removal (PBR) (Wade 1998, Slooten et al. 2006, to mitigate or minimize anthropogenic threats. In practice, the causes of incidental mortality are often generalized to fishery bycatch (Caswell et al. 1998, Read & Wade 2000, Dans et al. 2003, Lennert-Cody et al. 2004, Moore & Read 2008. The PBR calculation for cetaceans is based on an intrinsic population growth rate that is usually set as 0.04 when demographic data are not available (Wade 1998). ...
Predictive modeling of population trends can indicate the rate of population decline and risk of extinction, providing quantitative means of assessing conservation status and threats. Our study tests the rate of population change and risk of extinction of the Indo-Pacific humpback dolphin, Sousa chinensis, off Taiwan west coast; the only humpback dolphin population classified as Critically Endangered (CR) by the IUCN Red List of Threatened Species. Under the most optimistic assumptions, almost 60% of simulations (out of 250 replications 5000 iterations) predicted population decline exceeded 80% within three generations, while the mean estimate of population decline within one generation was > 50% of the current population numbers. Status classification under IUCN Red List Categories and Criteria Version 3.1 supported previous CR classification, while risk assessment that factored in anthropogenic impacts further increased the estimated extinction risk. With adult survival rate of 0.95, a modeled increment of annual bycatch rate by 1% of population size increased the probability of extinction within 100 years by 7.5%; although this increase was lower at higher adult survival rate. The estimated extinction risk was the greatest under the impact of habitat-loss, reaching a hazardous level when habitat carrying capacity dropped to less than 50%, indicating that habitat fragmentation and alteration of coastal environments pose the greatest threat to this population, even if the cumulative sum of fragmented patches of habitat may appear to be superficially large.
Previous studies indicate that the locations of the endangered loggerhead Caretta caretta and critically endangered leatherback Dermochelys coriacea sea turtles are influenced by water temperatures, and that incidental catch rates in the pelagic longline fishery vary by region. We present a Bayesian hierarchical model to examine the effects of environmental variables, including water temperature, on the number of sea turtles captured in the US pelagic longline fishery in the western North Atlantic. The modeling structure is highly flexible, utilizes a Bayesian model selection technique, and is fully implemented in the software program WinBUGS. The number of sea turtles captured is modeled as a zero-inflated Poisson distribution and the model incorporates fixed effects to examine region-specific differences in the parameter estimates. Results indicate that water temperature, region, bottom depth, and target species are all significant predictors of the number of loggerhead sea turtles captured. For leatherback sea turtles, the model with only target species had the most posterior model weight, though a re-parameterization of the model indicates that temperature influences the zero-inflation parameter. The relationship between the number of sea turtles captured and the variables of interest all varied by region. This suggests that management decisions aimed at reducing sea turtle bycatch may be more effective if they are spatially explicit.
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