Seabird‐induced natural mortality of forage fish varies with fish abundance: Evidence from five ecosystems

Abstract Contemporary stock assessment models used by fisheries management often assume that natural mortality rates are constant over time for exploited fish stocks. This assumption results in biased estimates of fishing mortality and reference points when mortality changes over time. However, it is difficult to distinguish changes in natural mortality from changes in fishing mortality, selectivity, and recruitment. Because changes in size structure can be indicate changes in mortality, one potential solution is to use population size-structure and fisheries catch data to simultaneously estimate time-varying natural and fishing mortality. Here we test that hypothesis, using a simulation experiment to test performance for four alternative estimation models that estimate natural and fishing mortality from size structure and catch data. We show that it is possible to estimate time-varying natural mortality in a size-based model, even when fishing mortality, recruitment, and selectivity are changing over time. Finally, we apply the model to North Sea sprat, and show that estimates of recruitment and natural mortality are similar to estimates from an alternative multispecies population model fitted to additional data sources. We recommend exploring potential trends in natural mortality in forage fish assessments using tools such as the one presented here.

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Towards a balanced presentation and objective interpretation of acoustic and trawl survey data, with specific reference to the eastern Scotian Shelf

Canadian Journal of Fisheries and Aquatic Sciences ◽

10.1139/cjfas-2016-0113 ◽

2016 ◽

Vol 73 (12) ◽

pp. 1914-1921 ◽

Cited By ~ 4

Author(s):

J. Michael Jech ◽

Ian H. McQuinn

Keyword(s):

Atlantic Cod ◽

Management Strategies ◽

Pelagic Fish ◽

Trophic Levels ◽

Fish Abundance ◽

Forage Fish ◽

Specific Reference ◽

Trawl Survey ◽

Scotian Shelf ◽

Eastern Canada

A debate has developed over the ecosystem consequences following the collapse of Atlantic cod throughout the coastal waters of eastern Canada. The explosive increase in pelagic fish abundance in scientific bottom-trawl catches on the eastern Scotian Shelf has been interpreted as being due to either (i) a “pelagic outburst” of forage fish abundance resulting from predator release or conversely (ii) a change in pelagic fish vertical distribution leading to a “suprabenthic habitat occupation” thereby increasing their availability to bottom trawls. These two interpretations have diametrically opposing ecological consequences and suggest different management strategies for these important forage fish species. We argue that an objective evaluation of the available evidence supports the hypothesis that the abundance of forage fish has not increased in response to the demise of cod and other top predators, and the reliance on a single sampling gear with low catchability has biased and will continue to bias the interpretation of demographic trends of pelagic fish populations. We advocate that multiple sampling technologies providing alternative perspectives are needed for the monitoring and management of the various trophic levels if we are to achieve a balanced and objective understanding of marine ecosystems.

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Natural mortality augments population fluctuations of forage fish

Fish and Fisheries ◽

10.1111/faf.12290 ◽

2018 ◽

Vol 19 (5) ◽

pp. 791-797 ◽

Cited By ~ 3

Author(s):

Nis S Jacobsen ◽

Timothy E Essington

Keyword(s):

Forage Fish ◽

Natural Mortality ◽

Population Fluctuations

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Evaluating impacts of forage fish abundance on marine predators

Conservation Biology ◽

10.1111/cobi.13709 ◽

2021 ◽

Author(s):

Christopher M. Free ◽

Olaf P. Jensen ◽

Ray Hilborn

Keyword(s):

Fish Abundance ◽

Forage Fish ◽

Marine Predators

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Estimation of natural mortality coefficient from fish abundance and catch data using Virtual Population Analysis (VPA)

Journal of Ocean University of China ◽

10.1007/s11802-007-0053-2 ◽

2007 ◽

Vol 6 (1) ◽

pp. 53-59

Author(s):

Yingbin Wang ◽

Qun Liu ◽

Yanjun Wang

Keyword(s):

Population Analysis ◽

Fish Abundance ◽

Natural Mortality ◽

Catch Data ◽

Virtual Population Analysis ◽

Virtual Population

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Contributions of adult mortality to declines of Puget Sound Pacific herring

ICES Journal of Marine Science ◽

10.1093/icesjms/fsx094 ◽

2017 ◽

Vol 75 (1) ◽

pp. 319-329 ◽

Cited By ~ 3

Author(s):

Margaret C Siple ◽

Andrew O Shelton ◽

Tessa B Francis ◽

Dayv Lowry ◽

Adam P Lindquist ◽

...

Keyword(s):

Age Structure ◽

Puget Sound ◽

Adult Mortality ◽

Forage Fish ◽

Pacific Herring ◽

Natural Mortality ◽

Structured Population Model ◽

Adult Age ◽

Age Structured

Abstract Forage fish undergo dramatic changes in abundance through time. Long-term fluctuations, which have historically been attributed to changes in recruitment, may also be due to changes in adult mortality. Pacific herring, a lightly exploited forage fish in Puget Sound, WA, have exhibited shifts in age structure and decreases in spawning biomass during the past 30 years. Here, we investigate changes in adult mortality as a potential explanation for these shifts. Using a hierarchical, age-structured population model, we indicate that adult natural mortality for Puget Sound Pacific herring has increased since 1973. We find that natural mortality has increased for every age class of adult (age 3+), especially age 4 fish, whose estimated mortality has doubled over the survey time period (from M = 0.84–1.76). We demonstrate that long-term shifts in mortality explain changes in age structure, and may explain biomass declines and failure to reach management thresholds for two spawning sites in Puget Sound (Cherry Point and Squaxin Pass). Temporal shifts in natural adult mortality could have negative implications for herring and herring predators. We demonstrate that adult mortality, in addition to recruitment variation, is an important driver for forage fish, which face exceptionally high natural mortality compared with other fishes.

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Forage fish abundance is a predictor of timing of breeding and hatching brood size in a coastal seabird

Marine Ecology Progress Series ◽

10.3354/meps11100 ◽

2015 ◽

Vol 519 ◽

pp. 209-220 ◽

Cited By ~ 6

Author(s):

SH Lorentsen ◽

T Anker-Nilssen ◽

KE Erikstad ◽

N Røv

Keyword(s):

Brood Size ◽

Fish Abundance ◽

Forage Fish ◽

Timing Of Breeding

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Community Ecology of Stream Fishes: Concepts, Approaches, and Techniques

Community Ecology of Stream Fishes: Concepts, Approaches, and Techniques ◽

10.47886/9781934874141.ch24 ◽

2010 ◽

Keyword(s):

Brown Trout ◽

Brook Trout ◽

Fish Assemblages ◽

Smallmouth Bass ◽

Fish Abundance ◽

Forage Fish ◽

Data Sets ◽

Biotic Factors ◽

Habitat Factors ◽

Habitat Variables

Abstract.—In regional survey studies of habitat and fish assemblages, potentially important biological interactions can be masked by strong gradients in habitat variables and associated collinearities among biological variables. We used structural equation modeling to compare the causal influences of local habitat and biotic factors on fish density in rivers and to determine the extent to which the set of sites chosen for analysis influenced their apparent importance. When all sites in our Michigan data set were used, spatial patterns in brook trout Salvelinus fontinalis biomass were 28 times more sensitive to habitat variables than brown trout Salmo trutta biomass. However, when the sample was restricted to trout streams, then brook trout biomass patterns were twice as sensitive to brown trout biomass as habitat variables. In a similar analysis for smallmouth bass Micropterus dolomieu, habitat factors had the strongest effects on fish densities when the analysis was based on all samples available. However, when the sample was limited to steams in which smallmouth bass actually occurred, direct effects of forage fish abundance and indirect effects of habitat via forage fish abundance were more prominent. In both the trout and smallmouth bass analyses, regional data sets (which included sites where the species of interest was absent) overemphasized the importance of habitat factors on fish abundance, but restricting the sample to sites having the species of interest elevated the importance of biotic factors. In reality, both habitat and biotic factors are important to these species, but the variance structure of the sample being analyzed had an overriding influence on the statistical importance of one versus the other. These findings help to resolve apparently conflicting results of previous studies assessing the relative influence of habitat and biotic factors on population abundance.

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