Carey T. Knight

A Potential New Energy Pathway in Central Lake Erie: the Round Goby Connection

Round gobies, invasive fish that entered Lake Erie in 1994, are altering energy, contaminant, and nutrient pathways. Our objective was to quantify how they alter energy pathways in the central basin of Lake Erie by describing their diet and identifying the degree to which predatory fish feed upon round gobies. We used bioenergetic models parameterized with data collected in the central basin between 1995 and 2002 to estimate the type and amount of prey eaten, the biomass accumulation rate for the round goby population, and a partitioning of the food energy into “new” energy derived from dreissenids as opposed to existing energy derived from zooplankton and non-dreissenid benthic prey. Mean (± SE) prey consumption peaked at 5.98 ± 2.17×104 tonnes wet mass in 1999 coincident with the maximum population size of 4.2 ± 1.5 billion round gobies. Zooplankton (40.2% by biomass) and dreissenid mussels (38.3%) dominated the prey consumed. Almost 90% of the zooplankton biomass was consumed by age-0 round gobies, while over 80% of the dreissenids were eaten by older ages. Standing stock biomass of round gobies ranged between 203 and 4,803 tonnes y−1 (interannual range), with an additional 475 to 8,943 tonnes of biomass accumulating through growth each year. Piscivorous fish showed an increasing reliance on round gobies as prey, with round gobies being the dominant prey fish in the diets of benthic-oriented predators. Hence, by being one of the few benthivores that exploit dreissenid mussels as prey, our analyses reveal that round gobies transfer new energy up the central Lake Erie food web.

The Round Goby (Neogobius melanostomus) Invasion: Current Research and Future Needs

Species native to Europe and Asia have invaded the Laurentian Great Lakes since the time of European settlement. The incidence of Eurasian invaders was accelerated in the 1840s with completion of the first passages by ocean-going vessels into the Great Lakes (Mills et al. 1993, 2000). The threat continues today primarily in the form of transoceanic cargo ships that can harbor Eurasian species within their ballast tanks. One of these species that invaded the Great Lakes from ballast transported from Eurasia is the round goby (Neogobius melanostomus), a small benthic fish native to the Black and Caspian seas. This special topic section is devoted to the round goby. Of the two species of gobiids, the tubenose goby (Proterorhinus marmoratus) and round goby, that invaded the Great Lakes in 1990, it is the round goby that has proliferated and spread farthest. The tubenose goby is limited in its distribution to the St.

A Computer Program for Age–Length Keys Incorporating Age Assignment to Individual Fish

Abstract We provide a computer program (AGEKEY) that quickly implements an age–length key approach for estimating fish population age structure and mean lengths at age. The programs 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 ver...

Genetic and morphometric differences demonstrate fine-scale population substructure of the yellow perch Perca flavescens: need for redefined management units

Whole-body morphometrics and 15 nuclear DNA microsatellite loci were analysed for 158 Perca flavescens collected during the spawning season from four spawning locations in central Lake Erie, two along the northern shore and two along the southern shore, to evaluate fine-scale variation (spanning 17-94 km). Results showed significant morphological and genetic differences among P. flavescens from the four locations. The magnitudes of differences were unrelated to geographic distance, demonstrating spatially heterogeneous levels of genetic divergence. These results linked morphometric and genetic variation, showing a discontinuity of scale between currently defined management units and population structure of P. flavescens in Lake Erie, and support that P. flavescens might exist as one or more metapopulations. Findings demonstrate the value of using complementary techniques for evaluating population structure.

Potential Effects of Jaw Tag Loss on Exploitation Estimates for Lake Erie Walleyes

Abstract 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 a...

Intercalibration of research survey vessels on Lake Erie

Abstract Fish abundance indices obtained from annual research trawl surveys are an integral part of fisheries stock assessment and management in the Great Lakes. It is difficult, however, to administer trawl surveys using a single vessel−gear combination owing to the large size of these systems, the jurisdictional boundaries that bisect the Great Lakes, and changes in vessels as a result of fleet replacement. When trawl surveys are administered by multiple vessel−gear combinations, systematic error may be introduced in combining catch-per-unit-effort (CPUE) data across vessels. This bias is associated with relative differences in catchability among vessel−gear combinations. In Lake Erie, five different research vessels conduct seasonal trawl surveys in the western half of the lake. To eliminate this systematic bias, the Lake Erie agencies conducted a side-by-side trawling experiment in 2003 to develop correction factors for CPUE data associated with different vessel−gear combinations. Correcting for syste...

Rapidly shifting maturation schedules following reduced commercial harvest in a freshwater fish

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.