Long-term Trend in Mean Density of Antarctic Krill (Euphausia superba) Uncertain

Summary

This study reexamines conflicting research findings on Antarctic krill population trends in the Southwest Atlantic. Previous analyses of the KRILLBASE dataset had produced contradictory results: one showing significant krill decline (particularly north of 60°S), while another found no significant decline. In this study, Candy reexamines the data using improved statistical methods called a meta-analytic linear mixed model (LMM) approach. The new analysis also incorporates nonlinear effects and climatological temperature data to get a more accurate picture.The improved analysis shows that while there may be some decline in krill density north of 60°S, the uncertainty is too high to draw firm conclusions. The statistical power was too weak to reliably detect even substantial population changes like a 70% decline between 1985-2005.The study concludes that current evidence neither strongly supports nor rejects the idea of dramatic krill decline in the Southwest Atlantic. This highlights how different statistical approaches can lead to very different conclusions about the same ecological data, emphasizing the importance of using appropriate methods and acknowledging uncertainty in environmental research.

Key Findings

Previous studies using the same KRILLBASE dataset reached opposite conclusions about krill population trends

Using improved statistical methods that account for measurement errors showed that the uncertainty in population trends is much higher than previously thought

There may be some decline in krill numbers north of 60°S, but with high statistical uncertainty

The statistical tests were too weak (only 5-31% chance) to reliably detect even a major 70% drop in krill populations

Climatological temperature was a significant predictor of krill density but didn't resolve the uncertainty in long-term trends

Abstract

Two recent attempts to model the long-term trend in mean density of Antarctic krill in the southwestern sector of the Atlantic using the KRILLBASE dataset using different statistical methods as well as inclusion versus exclusion of data from "non-scientific" nets have resulted in disparate conclusions. The approach that used a linear mixed model (LMM) fitted to the log of mean density, after standardisation was applied to individual net hauls and with means calculated for 12 spatial strata by years between 1976 and 2016, gave a highly statistically significant linear "regional" decline north of 60°S and, to a lesser degree, south of this latitude. The alternative approach that used a "hurdle" model fitted to the individual net haul data, excluded regional stratification, and excluded non-scientific nets failed to detect an overall significant decline. The method of modelling log transformed means was reappraised and corrected by applying a meta-analytic LMM approach. Additionally, nonlinear smooths in year by region and a smooth in mean "climatological temperature" were included in the LMM. This model showed on average a mostly consistent decline north of 60°S, however, neither trend was significantly different from a no-trend prediction with the trend north of 60°S highly uncertain. Uncertainty of predictions resulted in only weak power to detect a substantial decline of the order of 70% between 1985 and 2005. These model-based inferences neither strongly support nor reject a general hypothesis that there has been a dramatic decline in density of Antarctic krill in the Southwest Atlantic over this period.

Published in

Annual Research & Review in Biology

2021

Authors

Steven G. Candy

Institutions

S Candy Statistical Modelling Pty Ltd

Methods

Data

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