Lipid concentration and fatty acid composition of common prey species or taxonomic groups from four distinct regions of Lake Michigan were quantified (n = 894). We used a combination of parametric and nonparametric statistics to assess the differences in fatty acid signatures (FAS) among species and to evaluate intraspecies variation relative to interspecies variation in FAS. Discriminant function analysis performed on 13 species or taxa groups using the 18 most abundant fatty acids revealed clear separation among taxa, with overall classification success reaching 89%. Species were readily distinguished based on their overall fatty acid profile in spite of intraspecies variation (temporal, regional, and size-related). Among species sampled, pelagic and benthic clusters were formed based on the degree of fatty acid profile similarity. In alewife ( Alosa pseudoharengus ) and round goby ( Neogobius melanostomus ), fatty acid compositions differed with fish size, sampling location, and temporal variation; however, the magnitude of these differences was small relative to differences between species. Our results demonstrate the utility of fatty acid signatures in studies of food webs in large freshwater ecosystems. This study is also a necessary first step toward development of mechanistic research that investigates the effects of variation in fatty acids within the prey base on top predators.

Lake Trout Salvelinus namaycush were extirpated from Lake Michigan by the early 1950s, and as part of an effort to restore naturally reproducing populations, hatchery‐reared fish have been stocked since the early 1960s. Stocked fish are marked with a fin clip to differentiate them from wild, lake‐produced Lake Trout; marking error for the 2007–2010 year‐classes of Lake Trout stocked by federal hatcheries averaged 3.0%. Egg deposition, emergent fry, and wild juvenile Lake Trout have previously been observed, but no sustained wild recruitment has been measured in assessment surveys or in sport and commercial fishery catches. In 2011 and 2012, we caught juvenile Lake Trout in gill‐net and bottom trawl catches that were targeting Bloater Coregonus hoyi in water depths greater than 80 m. Unclipped, wild Lake Trout represented 20% of all Lake Trout caught in a southern offshore region of Lake Michigan. In northwestern Lake Michigan wild recruits represented from 10% to 27% of the 2007–2009 year‐classes, and we recovered a small number of wild Lake Trout from the 2010 year‐class. This is the first evidence for consecutive year‐classes of naturally produced Lake Trout surviving beyond the fry stage in Lake Michigan.Received July 13, 2012; accepted November 26, 2012

No abstract

Relationships among otolith weight and age have been explored widely as cost‐effective means to predict age. However, otolith weight is influenced by both fish age and somatic growth, making it necessary to partition these confounding effects to best use otolith weight to predict age. We used several hatchery strains of Lake Trout Salvelinus namaycush that varied in capture season, year, location, and mean size at stocking to develop a maximum likelihood model of otolith weight as complementary but independent functions of age (mg/year) and fish length (mg/mm). Once these relationships were established, we determined age probabilities for each fish based on their measurements. Our best model included age and somatic growth components, with cumulative mean temperature as a variable to scale for seasonal differences in otolith accretion. For fish between 3 and 19 years of age, otolith weight increased by 1.16 mg/year, with an additional somatic growth component of 0.62 × exp(fish length × 0.0038) mg/mm. We correctly assigned age to 60% of the age‐3 fish, but this decreased to about 10% among the oldest age‐classes; however, predicted age residuals approximated normal distributions within each age‐class. The model was robust to seasonal and annual variation, but somatic growth differences among some strain and stocking‐size groupings resulted in modal aging biases of ±1 year. Given the stability in otolith weight components arising from fish age and somatic growth, we suggest that modeling otolith weight using reader age estimates offers a new quality assurance/quality control tool to assess whether reader ages support the widely documented properties of otolith growth. When otolith weight and fish length are more informative of age, our model may also be useful in estimating age composition data similar to an age–length key. Received December 19, 2016; accepted March 20, 2017 Published online May 26, 2017

Fatty acids (FA) are increasingly being used in ecology to qualitatively infer diets of consumers. Analysis of FA data requires standardization to express FAs either in mg g−1 lipids, mg g−1 tissue, or as percentages of the total mass of FAs. Additionally, various transformations [square root, arcsin, log(X + 1), log‐ratio, etc.] are often used to differentially weight the contribution of less abundant FAs. The choice of standardization unit and transformation can affect interpretations of results and ultimately our understanding of trophic relationships. Data from published feeding experiments were analyzed with visualization (i.e., nMDS) and multivariate rank‐based methods (i.e. Mantel Tests) to evaluate how choice of standardization, transformation, and resemblance metric (i.e., Euclidean distance, Bray–Curtis similarities, etc.) reflects known dietary treatment groups. Our results indicate that diet interpretations were best inferred from data standardized to the total mass of FAs quantified. We found transforming data provided only weak advantages for discriminating among diet groups. Euclidean distances between FA proportions represented the known dietary differences to a high degree, are relatively easily interpretable, are applicable to a wide variety of statistical techniques, and are thus a reasonable choice of metric when analyzing FA proportions.