Sea otter mortality in fish and shellfish traps: estimating potential impacts and exploring possible solutions

Hatfield, Brian B.; Ames, Jack A.; Estes, James A.; Tinker, M. Tim; Johnson, Andrew B.; Staedler, Michelle M.; Harris, Michael D.

doi:10.3354/esr00327

Cited by 20 publications

(17 citation statements)

References 29 publications

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“…In the past, anthropogenic trauma, including fishing gear entanglement and gunshot, has been an important source of mortality for sea otters, and was responsible for a period of population decline from the mid-1970s through the early 1980s (Wendell et al 1986, Herrick and Hanan 1988, Estes et al 2003, Hatfield et al 2011. In the past, anthropogenic trauma, including fishing gear entanglement and gunshot, has been an important source of mortality for sea otters, and was responsible for a period of population decline from the mid-1970s through the early 1980s (Wendell et al 1986, Herrick and Hanan 1988, Estes et al 2003, Hatfield et al 2011.…”

Section: Discussionmentioning

confidence: 99%

Gaps in kelp cover may threaten the recovery of California sea otters

et al. 2018

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Despite more than a century of federal protection, the California sea otter Enhydra lutris nereis remains threatened under the U.S. Endangered Species Act (ESA), and the population has not appreciably expanded its range in two decades. Here, we examine a novel dataset of 725 sea otter live strandings from 1984-2015 to gain insights into demographic and environmental factors underlying threats to sea otter recovery. Using multinomial logistic regression to evaluate spatiotemporal patterns of stranding causes, we demonstrate that increases in stranding rates, particularly outside the range center, are related to a substantial increase in shark bites. By contrast, trauma linked to human activities has declined dramatically, and now accounts for less than 5% of stranding cases. Within the range core, where the sea otter population seems regulated by prey availability, symptoms of energetic stress represent more than 63% of all strandings and are strongly associated with high sea otter density. Conversely, in range peripheries, the majority of strandings are caused by shark bite and neurological disease. Notably, these threats are virtually absent where nearshore habitat is characterized by at least 10% kelp canopy cover. Our analyses reveal that declining kelp cover may therefore constrain the population's spatial expansion and recovery in two key ways. Absence of kelp intensifies density-independent threats in the range peripheries, and likely limits dispersal of reproductive females, which depend on kelp canopy for nursery habitat. These results highlight the significance of both top-down and bottomup processes in population dynamics, and inform an ecosystem-based approach to conservation planning.

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Section: Discussionmentioning

confidence: 99%

Gaps in kelp cover may threaten the recovery of California sea otters

et al. 2018

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“…The density at which populations reach equilibrium ( K ′, in units of otters/km 2 of habitat) varies depending on local habitat characteristics and prey productivity (Laidre et al , ; Burn et al ; Gregr et al ). In addition to density‐dependent variation in growth, trends in abundance are affected by dispersal between areas, influences of non‐density‐dependent mortality factors such as predation (Estes et al ), fishing entanglement (Hatfield et al ), or harvest mortality (Bodkin and Ballachey ), and stochastic variation in annual mortality caused by factors such as infectious disease (Goldstein et al ). Our process model incorporates all these features.…”

Section: Methodsmentioning

confidence: 99%

Trends and Carrying Capacity of Sea Otters in Southeast Alaska

et al. 2019

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Sea otter populations in Southeast Alaska, USA, have increased dramatically from just over 400 translocated animals in the late 1960s to >8,000 by 2003. The recovery of sea otters to ecosystems from which they had been absent has affected coastal food webs, including commercially important fisheries, and thus information on expected growth and equilibrium abundances can help inform resource management. We compile available survey data for Southeast Alaska and fit a Bayesian state‐space model to estimate past trends and current abundance. Our model improves upon previous analyses by partitioning and quantifying sources of estimation error, accounting for over‐dispersion of aerial count data, and providing realistic measurements of uncertainty around point estimates of abundance at multiple spatial scales. We also provide estimates of carrying capacity (K) for Southeast Alaska, at regional and sub‐regional scales, and analyze growth rates, current population status and expected future trends. At the regional scale, the population increased from 13,221 otters in 2003 to 25,584 otters in 2011. The average annual growth rate in southern Southeast Alaska (7.8%) was higher than northern Southeast Alaska (2.7%); however, growth varied at the sub‐regional scale and there was a negative relationship between growth rates and the number of years sea otters were present in an area. Local populations vary in terms of current densities and expected future growth; the mean estimated density at K was 4.2 ± 1.58 sea otters/km2 of habitat (i.e., the sub‐tidal benthos between 0 m and 40 m depth) and current densities correspond on average to 50% of projected equilibrium values (range = 1–97%) with the earliest‐colonized sub‐regions tending to be closer to K. Assuming a similar range of equilibrium densities for currently un‐occupied habitats, the projected value of K for all of Southeast Alaska is 74,650 sea otters. Future analyses can improve upon the precision of K estimates by employing more frequent surveys at index sites and incorporating environmental covariates into the process model to generate more accurate, location‐specific estimates of equilibrium density. © 2019 The Authors. The Journal of Wildlife Management Published by Wiley Periodicals, Inc.

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“…; Hatfield et al . ). For example, organisms were caught in the field and then brought back to the laboratory for monitoring delayed impacts (Rutecki and Meyers ; Leland et al .…”

Section: Reviewmentioning

confidence: 97%

“…) and sea otter ( Enhydra lutris, Mustelidae; Hatfield et al . ). Other changes to the mesh size (Dieterman et al .…”

Section: Reviewmentioning

confidence: 97%

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Mitigating unaccounted fishing mortality from gillnets and traps

2013

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Gillnets and traps often are considered to have fewer holistic environmental impacts than active fishing gears. However, in addition to the targeted catches, gillnets and traps still cause unwanted mortalities due to (i) discarding, (ii) ghost fishing of derelict gear, (iii) depredation, (iv) escaping or dropping out of gear, (v) habitat damage, and potentially (vi) avoiding gear and predation and (vii) infection of injuries sustained from most of the above. Population‐level concerns associated with such ‘unaccounted fishing mortalities’ from gillnets and traps have been sufficient to warrant numerous attempts at mitigation. In this article, we reviewed relevant research efforts, locating 130 studies in the primary literature that concomitantly quantified mortalities and their resolution through technical modifications, with the division of effort indicating ongoing concerns. Most studies (85) have focused on discard mortality, followed by ghost‐fishing (24), depredation (10) and escape (8) mortalities. The remaining components have been poorly studied (3). All problematic mortality components are affected by key biological (e.g. species), technical (e.g. fishing mechanisms) and/or environmental (e.g. temperature) factors. We propose that these key factors should be considered as part of a strategy to reduce impacts of these gears by first assessing modifications within and then beyond conventional configurations, followed by changes to operational and handling practices. Justification for this three‐tiered approach is based not only on the potential for cumulative reduction benefits, but also on the likely ease of adoption, legislation and compliance.

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Sea otter mortality in fish and shellfish traps: estimating potential impacts and exploring possible solutions

Cited by 20 publications

References 29 publications

Gaps in kelp cover may threaten the recovery of California sea otters

Gaps in kelp cover may threaten the recovery of California sea otters

Trends and Carrying Capacity of Sea Otters in Southeast Alaska

Mitigating unaccounted fishing mortality from gillnets and traps

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