Hui-Yu Wang

Movement of Walleyes in Lakes Erie and St. Clair Inferred from Tag Return and Fisheries Data

Abstract Lake Erie walleyes Sander vitreus support important fisheries and have been managed as one stock, although preliminary tag return and genetic analyses suggest the presence of multiple stocks that migrate among basins within Lake Erie and into other portions of the Great Lakes. We examined temporal and spatial movement and abundance patterns of walleye stocks in the three basins of Lake Erie and in Lake St. Clair with the use of tag return and sport and commercial catch-per-unit effort (CPUE) data from 1990 to 2001. Based on summer tag returns, western basin walleyes migrated to the central and eastern basins of Lake Erie and to Lake St. Clair and southern Lake Huron, while fish in the central and eastern basins of Lake Erie and in Lake St. Clair were primarily caught within the basins where they were tagged. Seasonal changes in sport and commercial effort and CPUE in Lake Erie confirmed the walleye movements suggested by tag return data. Walleyes tagged in the western basin but recaptured in the ...

Eco-genetic model to explore fishing-induced ecological and evolutionary effects on growth and maturation schedules

Eco‐genetic individual‐based models involve tracking the ecological dynamics of simulated individual organisms that are in part characterized by heritable parameters. We developed an eco‐genetic individual‐based model to explore ecological and evolutionary interactions of fish growth and maturation schedules. Our model is flexible and allows for exploration of the effects of heritable growth rates (based on von Bertalanffy and biphasic growth patterns), heritable maturation schedules (based on maturation reaction norm concepts), or both on individual‐ and population‐level traits. In baseline simulations with rather simple ecological trade‐offs and over a relatively short time period (<200 simulation years), simulated male and female fish evolve differential genetic growth and maturation. Further, resulting patterns of genetically determined growth and maturation are influenced by mortality rate and density‐dependent processes, and maturation and growth parameters interact to mediate the evolution of one another. Subsequent to baseline simulations, we conducted experimental simulations to mimic fisheries harvest with two size‐limits (targeting large or small fish), an array of fishing mortality rates, and assuming a deterministic or stochastic environment. Our results suggest that fishing with either size‐limit may induce considerable changes in life‐history trait expression (maturation schedules and growth rates), recruitment, and population abundance and structure. However, targeting large fish would cause more adverse genetic effects and may lead to a population less resilient to environmental stochasticity.

Changes in size, age, and sex ratio composition of Pacific bluefin tuna (Thunnus orientalis) on the northwestern Pacific Ocean spawning grounds

&NA; The Pacific bluefin tuna (Thunnus orientalis, PBF), an iconic fisheries species, is heavily exploited and its biomass is at a historically low level. To enhance effectiveness of management and conservation, a study was initiated to examine the demographic changes of the PBF between 2002 and 2015. Annual tuna age compositions were estimated by applying an empirical age‐length key, based on the ages and lengths of 892 fish, to the length frequencies observed among annual catches in the northwestern Pacific spawning grounds located near Taiwan. Ages of the spawners concentrated in 7‐20‐years old (range = 4‐28 years), corresponding to fork lengths between 200 and 240 cm. The size and age compositions were influenced by the strong 1994 and 1996 year‐cohorts, which showed a relatively high abundance in most years except 2007‐2009. Between 2013 and 2015, recruits of 2005‐2009 year‐cohorts (age 6‐10) increased in relative abundance while the strong 1994 and 1996 year‐cohorts decreased as they neared the end of their lifespan. PBF smaller than 200 cm were rarely found in the tropical areas and the mean sizes of the PBF caught south of 24°N were significantly larger than those caught north of 24°N. These results suggested that the choice of spawning latitudes of the PBF was influenced by the size of spawning adults. The sex ratio of smaller (<200 cm) and intermediate size classes (200‐230 cm) was female biased (50‐75%) while that of larger fish (>240 cm) were male biased (62‐100%). The size and age composition of the PBF was mainly influenced by the recruitment of young spawners and was less sensitive to the catch of longline fisheries. However the impact of longline fisheries on the reproductive potential of this species warrants further investigation.

Fishing‐induced changes in adult length are mediated by skipped‐spawning

Elucidating fishing effects on fish population dynamics is a critical step toward sustainable fisheries management. Despite previous studies that have suggested age or size truncation in exploited fish populations, other aspects of fishing effects on population demography, e.g., via altering life histories and density, have received less attention. Here, we investigated the fishing effects altering adult demography via shifting reproductive trade-offs in the iconic, overexploited, Pacific bluefin tuna Thunnus orientalis. We found that, contrary to our expectation, mean lengths of catch increased over time in longline fisheries. On the other hand, mean catch lengths for purse seine fisheries did not show such increasing trends. We hypothesized that the size-dependent energetic cost of the spawning migration and elevated fishing mortality on the spawning grounds potentially drive size-dependent skipped spawning for adult tuna, mediating the observed changes in the catch lengths. Using eco-genetic individual-based modeling, we demonstrated that fishing-induced evolution of skipped spawning and size truncation interacted to shape the observed temporal changes in mean catch lengths for tuna. Skipped spawning of the small adults led to increased mean catch lengths for the longline fisheries, while truncation of small adults by the purse seines could offset such a pattern. Our results highlight the eco-evolutionary dynamics of fishing effects on population demography and caution against using demographic traits as a basis for fisheries management of the Pacific bluefin tuna as well as other migratory species.