Anthropogenic change decouples a freshwater predator's density feedback.

Intraspecific interactions within predator populations can affect predator-prey dynamics and community structure, highlighting the need to better understand how these interactions respond to anthropogenic change. To this end, we used a half-century (1969-2018) of abundance and size-at-age data from Lake Erie's walleye (Sander vitreus) population to determine how anthropogenic alterations have influenced intraspecific interactions. Before the 1980s, the length-at-age of younger walleye (ages 1 and 2) negatively correlated with older (age 3 +) walleye abundance, signaling a 'density feedback' in which intraspecific competition limited growth. However, after the early 1980s this signal of intraspecific competition disappeared. This decoupling of the density feedback was related to multiple anthropogenic changes, including a larger walleye population resulting from better fisheries management, planned nutrient reductions to improve water quality and transparency, warmer water temperatures, and the proliferation of a non-native fish with novel traits (white perch, Morone americana). We argue that these changes may have reduced competitive interactions by reducing the spatial overlap between older and younger walleye and by introducing novel prey. Our findings illustrate the potential for anthropogenic change to diminish density dependent intraspecific interactions within top predator populations, which has important ramifications for predicting predator dynamics and managing natural resources.

Utilization of stomach content DNA to determine diet diversity in piscivorous fishes.

The objective of the study was to validate and apply DNA-based approaches to describe fish diets. Laboratory experiments were performed to determine the number of hours after ingestion that DNA could be reliably isolated from stomach content residues, particularly with small prey fishes (c. 1 cm, <0·75 g). Additionally, experiments were conducted at different temperatures to resolve temperature effects on digestion rate and DNA viability. The molecular protocol of cloning and sequencing was then applied to the analysis of stomach contents of wild fishes collected from the western basin of Lake Erie, Canada-U.S.A. The results showed that molecular techniques were more precise than traditional visual inspection and could provide insight into diet preferences for even highly digested prey that have lost all physical characteristics.