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We provide a computer program (AGE-KEY) that quickly implements an age-length key approach for estimating fish population age structure and mean lengths at age. The program's approach to age assignment differs from previous approaches in that ages are explicitly assigned to individual unaged fish. Because individual age assignment is necessary to describe the variation around estimates of mean catch per effort (CPE) at age and to compare these estimates through standard statistical procedures, the number of fish of a specified age captured in each unit of effort must be known or estimated. Individual age assignment can be confounded by instances of fractionality, where ages cannot be evenly assigned to fish within individual length intervals. Program AGEKEY resolves the problem of fractionality by applying an integer-based approach to age assignment. When we tested the validity of the program with two sample data sets, AGEKEY yielded estimates of age frequency and mean lengths at age that were very similar to estimates calculated through spreadsheet approaches. Program AGEKEY can quickly derive estimates of age frequency and mean lengths at age using an age-length key approach that will provide users with the data necessary to describe the precision associated with estimates of CPE at age and allow for statistical comparisons of CPE at age across surveys.

Seasonal degradation of aquatic habitats from hypoxia occurs in numerous freshwater and coastal marine systems and can result in direct mortality or displacement of fish. Yet, fishery landings from these systems are frequently unresponsive to changes in the severity and extent of hypoxia, and population-scale effects have been difficult to measure except in extreme hypoxic conditions with hypoxia-sensitive species. We investigated fine-scale temporal and spatial variability in dissolved oxygen in Lake Erie as it related to fish distribution and catch efficiencies of both active (bottom trawls) and passive (trap nets) fishing gears. Temperature and dissolved oxygen loggers placed near the edge of the hypolimnion exhibited much higher than expected variability. Hypoxic episodes of variable durations were frequently punctuated by periods of normoxia, consistent with high-frequency internal waves. High-resolution interpolations of water quality and hydroacoustic surveys suggest that fish habitat is compressed during hypoxic episodes, resulting in higher fish densities near the edges of hypoxia. At fixed locations with passive commercial fishing gear, catches with the highest values occurred when bottom waters were hypoxic for intermediate proportions of time. Proximity to hypoxia explained significant variation in bottom trawl catches, with higher catch rates near the edge of hypoxia. These results emphasize how hypoxia may elevate catch rates in various types of fishing gears, leading to a lack of association between indices of hypoxia and fishery landings. Increased catch rates of fish at the edges of hypoxia have important implications for stock assessment models that assume catchability is spatially homogeneous.

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Size-selective harvest of fish stocks can lead to maturation at smaller sizes and younger ages, which may depress stock productivity and recovery. Such changes in maturation may be very slow to reverse, even following complete fisheries closures. We evaluated temporal trends in maturation of five Great Lakes stocks of yellow perch (Perca flavescens Mitchill) using indices that attempt to disentangle plastic and evolutionary changes in maturation: age at 50% maturity and probabilistic maturation reaction norms (PMRNs). Four populations were fished commercially throughout the time series, while the Lake Michigan fishery was closed following a stock collapse. We documented rapid increases in PMRNs of the Lake Michigan stock coincident with the commercial fishery closure. Saginaw Bay and Lake Huron PMRNs also increased following reduced harvest, while Lake Erie populations were continuously fished and showed little change. The rapid response of maturation may have been enhanced by the short generation time of yellow perch and potential gene flow between northern and southern Lake Michigan, in addition to potential reverse adaptation following the fishing moratorium. These results suggest that some fish stocks may retain the ability to recover from fisheries-induced life history shifts following fishing moratoria.

We used an individual-based bioenergetic model to simulate the phosphorus flux of the round goby (Neogobius melanostomus) population in central Lake Erie during 1995–2002. Estimates of round goby diet composition, growth rates, and population abundance were derived from field sampling. As an abundant introduced fish, we predicted that round gobies would influence phosphorus cycling both directly, through excretion, and indirectly, through consumption of dreissenid mussels, whose high mass-specific phosphorus excretion enhances recycling. In 1999, when age-1+ round gobies reached peak abundance near 350 million (2.4 kg·ha–1), annual phosphorus excretion was estimated at 7 t (1.4 × 10–3 mg P·m–2·day–1). From an ecosystem perspective, however, round gobies excreted only 0.4% of the phosphorus needed by the benthic community for primary production. Indirectly, round gobies consumed <0.2% of dreissenid population biomass, indicating that round gobies did not reduce nutrient availability by consuming dreissenids. Compared with previous studies that have revealed introduced species to influence phosphorus cycling, round gobies likely did not attain a sufficiently high biomass density to influence phosphorus cycling in Lake Erie.

We used mark-recapture data collected from 1992 to 1995 to evaluate the potential effects of jaw tag loss on exploitation (u) estimates for walleyes Sander vitreus migrating from Lake Erie into the Grand River, Ohio. Loss of small jaw tags (size 10) detected during tagging periods was less than 17%. The annual loss rate of small tags exceeded 49% during each of the 2 years in which they were used. We detected no loss of large tags (size 12) during tagging periods, and annual rates of large-tag loss ranged from 23% to 50%. Exploitation rates adjusted for tag loss ranged from 14% to 39%, and unadjusted estimates ranged from 7% to 20%. Failure to adjust for annual tag loss rates resulted in 25-59% underestimation of u; however, error rates may be inflated due to a lack of knowledge concerning temporal patterns in tag loss. Our findings indicate that jaw tag loss rates must be accounted for when estimating population parameters such as u from mark-recapture studies. Furthermore, if agency personnel are able to analyze large samples of harvested fish for tags, the use of internal tag types that typically exhibit higher retention rates than jaw tags could reduce tag loss and eliminate biases due to nonreporting.