Elsevier

Fisheries Research

Volume 236, April 2021, 105825
Fisheries Research

Spatio-temporal model reduces species misidentification bias of spawning eggs in stock assessment of spotted mackerel in the western North Pacific

https://doi.org/10.1016/j.fishres.2020.105825Get rights and content
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Abstract

Species identification based on morphological characteristics includes species misidentification, leading to estimation bias in stock assessment and posing challenges difficult to be resolved. The spawning eggs of spotted mackerel Scomber australicus and chub mackerel S. japonicus in the western North Pacific are used for stock assessment as an index of spawning biomass and are classified based on egg diameter by past evidence. However, the difference in the distribution of egg diameters between the two species has become so ambiguous that the spawning eggs of chub mackerel may be classified as spotted mackerel. This can be explained by the larger distribution of egg diameters in chub mackerel with increasing stock abundance, resulting in overlap with the distribution of egg diameters in spotted mackerel. This leads to species misidentification and biased estimates of spotted mackerel abundance. To overcome this bias, it is necessary to develop a standardization method to remove the effect of species misidentification. Here, we demonstrate that a recently-developed spatio-temporal model can easily and efficiently reduce estimation bias for egg density and stock abundance in the spotted mackerel, using 15 years data for spawning eggs. We incorporated species identification error as the effect of the egg density of chub mackerel on the catchability of spotted mackerel in the spatio-temporal model. The index estimated from the model decreased temporal fluctuation substantially. When using the index accounting for species misidentification, the retrospective bias of abundance estimates for spotted mackerel decreased by about half compared with using the indices that ignored species misidentification. These results suggest that incorporating species misidentification bias is an essential process for improving stock assessment.

Keywords

Species identification
Stock assessment
Retrospective bias
Small pelagic fish

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