Sources of variation in stomach contents of predators of Atlantic herring in the Northwest Atlantic during 1973–2014

Spatial and temporal variation in stomach-contents data is often unquantified or combined in such a way (e.g. averaged among years) that true signal in diets may be lost. Using a delta approach, this paper fits generalized additive mixed models (GAMMs) to the amount of Atlantic herring (Clupea harengus) identified in predator stomachs using only data from stomachs in which herring occurred, and to the probability that a stomach contained herring. Both the amount of herring in stomachs and the probability of a stomach containing herring varied seasonally, spatially, and among years. Of the random effects in each GAMM, the effect of predator species had the largest variance. An index of herring abundance derived from the stomach-contents data was generally consistent with recent herring stock assessments. The temporal and spatial variation in the stomach-contents data suggested that the effect of averaging or combining stomach-contents data among years, seasons, or areas may lead to falsely precise or biased estimates from multispecies assessments or in estimates of consumption, and may restrain the relevance of static foodweb models.

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title: Seasonal Variation in Guild Structure of the Puget Sound Demersal Fish Community
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title: The foraging ecology of larval and juvenile fishes
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title: Fish gut content analysis: robust measures of diet composition
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approaches (i.e. ‘bulk’ methods measuring biomass or volume of each prey type)
and suggest that frequency of occurrence (%F) provides the most robust and interpretable measure of diet composition. The additional information provided by bulk
methods contains unquantiﬁable and potentially signiﬁcant error from a variety of
sources. In our experience, the contents of most guts cannot be unambiguously
separated into prey categories for quantiﬁcation because of the presence of unidentiﬁable and inseparable partially digested material. Even where separation is possible, the composition of a gut at one point in time is affected by many
unquantiﬁable factors unrelated to the actual composition of the diet. Consequently, bulk methods provide ambiguous interpretations from superﬁcially quantitative models. Where research questions require more detail, these problems mean
there is little alternative to time-consuming approaches like prey reconstruction.
However, for the descriptions of dietary composition presented in many studies, %
F provides robust data that overcome many of the limitations of the more
detailed approaches and provides considerable logistical and economic beneﬁts.

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title: An Ecosystem Approach for Assessment Advice and Biological Reference Points for the Gulf of Maine-Georges Bank Atlantic Herring Complex
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title: The advantage of explicitly incorporating predation mortality into age-structured stock assessment models: an application for Atlantic mackerel
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title: Stable and Efficient Multiple Smoothing Parameter Estimation for Generalized Additive Models
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title: Fast stable direct fitting and smoothness selection for generalized additive models
abstract: Existing computationally efficient methods for penalized likelihood generalized additive model fitting employ iterative smoothness selection on working linear models (or working mixed models). Such schemes fail to converge for a non-negligible proportion of models, with failure being particularly frequent in the presence of concurvity. If smoothness selection is performed by optimizing 'whole model' criteria these problems disappear, but until now attempts to do this have employed finite-difference-based optimization schemes which are computationally inefficient and can suffer from false convergence. The paper develops the first computationally efficient method for direct generalized additive model smoothness selection. It is highly stable, but by careful structuring achieves a computational efficiency that leads, in simulations, to lower mean computation times than the schemes that are based on working model smoothness selection. The method also offers a reliable way of fitting generalized additive mixed models.

title: Evaluating the sensitivity of a trophic mass- balance model (Ecopath) to imprecise data inputs
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title: Predator size - prey size relationships of marine fish predators: interspecific variation and effects of ontogeny and body size on trophic-niche breadth
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