While predation is widely accepted as a major cause of mortality for fish larvae, its extent is largely unknown because few studies have been able to identify larvae in the stomach contents of predatory fish. Rapid digestion rates probably explain why fish larvae are rarely found in stomach contents, yet quantification of digestion rates of fish larvae is generally lacking, especially in freshwater systems. Using a series of laboratory experiments, we quantified the effects of temperature and larval fish (prey) size on digestion rate. We also evaluated whether species type (both predator and prey) influences digestion rate and described the morphological breakdown of fish larvae during digestion. Bluegills Lepomis macrochirus and yellow perch Perca flavescens were force-fed the larvae of guppies Poecilia spp., rainbow trout Oncorhynchus mykiss, and yellow perch at a range of temperatures (7-228C), and digestion rates were measured using prey mass before and after digestion (i.e., proportional loss of prey mass after ingestion). As expected, digestion rates increased with water temperature and decreased with prey body mass but were unaffected by species of predator. A confounding effect of prey type (fresh versus frozen) prevented a thorough evaluation of prey species, although yellow perch and rainbow trout (both previously flash-frozen) were digested at similar rates. The complete breakdown of larvae in predator stomachs and the loss of morphological characters needed to identify larvae occurred rapidly, confirming the challenges of evaluating predation mortality based on stomach contents of field-collected predators. Ultimately, our findings can be used to help researchers quantify the likelihood of detecting larval fish in the stomachs of field-caught predators when using conventional stomach content analyses.

No abstract

We produced a spatially explicit assessment of the changes in stocking, catch, fishing effort, and catch per effort (CPE) for Chinook Salmon Oncorhynchus tshawytscha in Lakes Michigan and Huron from 1986 to 2011. We focused on describing spatial differences in the changes that occurred during three well‐known episodes of rapid change: (1) a decline in abundance during 1986–1994 in Lake Michigan, (2) a recovery in abundance during 1994–2006 in Lake Michigan, and (3) a decline in abundance during 2002–2010 in Lake Huron. We used a spatial grid system to describe and contrast trends in fishing effort and CPE among the main lake basins (Michigan, Huron, and Georgian) and subregions within those basins. We applied linear regressions, ANCOVAs, and Tukey's tests to assess differences. We found that trends differed among and within basins during all three episodes, which resulted in changes in the distribution of fishing effort and CPE. Fishing effort generally decreased in all basins and subregions over the entire 25 years, but it decreased less in areas where CPE had increased. The timing of the recovery episode and second mortality episode overlapped, so CPE simultaneously increased from 79 to 139 fish/1,000 h of fishing in the Michigan basin and decreased from 65 to 35 fish/1,000 h of fishing in the Huron basin. Movement of fishing effort and Chinook Salmon from the Huron basin to the Michigan basin probably occurred during this time. The CPE did not change significantly in the Georgian basin. Within basins, CPE exhibited sharp declines of more than 80% in some subregions during both mortality episodes but declined much less or not at all in others. After the recovery episode, areas of highest CPE in the Michigan basin had shifted from eastern subregions to western subregions. Received February 2, 2016; accepted April 23, 2016 Published online August 30, 2016

No abstract