Mortality of Yellow Perch (Perca flavescens) due to Confinement and Handling in the Drop Net Fishery in Southern Green Bay, Lake Michigan

Abstract

Net confinement and catch sorting procedures associated with the drop net segment of the southern Green Bay commercial fishery for yellow perch resulted in substantial non harvest mortality. Catches of yell ow perch taken in 3-day sets (the maximum legal set duration) averaged 7.7% dead perch in 1982 and 6.0% in 1983. Sets extended 1 to 8 days beyond the 3-day legal limit resulted in catches averaging 69% {S.D. = 31) dead perch. Thirty-six percent of 2,592 sublegal perch handled by commercial methods died within 24 hours. ANOCOV of standard drop net catches from 1982 and 1983 indicated that closing the commercial perch fishing season during June 1983 substantially decreased yellow perch mortality during that period. A multiple regression equation was developed which explained 61% of the variation in perch mortality for 28 extended set experiments. The equation is arsin %DEAD = 8.0 (HOLD) - 4.7 (D.O.) + 1.9 (TEMP) - 28.0, where HOLD = time (days) that perch were held after a 3-day set; D.O. = dissolved oxygen (mg/1) at net depth; and TEMP = water temperature at net depth. A second multiple regression equation was developed that explained 63% of the variation in sublegal perch (SLP) mortality for 48 handling experiments. The equation is %SLP = 2.07 (E.T.) + 2.37 (HAND) + 3.8 (DECK) - 96.07, where E.T. = elapsed time (hrs.) after handling; HAND = handling time (mins.); and DECK= deck temperature (°C). Modified Ricker yield models predicted that minimum length limits (MLLs) and mesh size regulations for drop nets could be used to reduce handling and net mortality of yellow perch without reducing long-term yield to commercial fishermen. I developed five yield models, designated I-V, based on drop nets with mesh sizes of 48, 51, 54, 57, and 60 mm respectively. Each model was divided into four submodels, A-D, based on MLLs of 191, 203, 216, and 229 mm. I think the best compromise between yield and reductions in mortality of perch was predicted for Model IVA which combined a 191-mm MLL with 57-mm mesh nets. Model IVA predicted a 38% increase in yield, a 96% decrease in handling mortality, and 24% decrease in net mortality, per 1000 Kg of age 1 recruits, compared with 1982-83 conditions as simulated by Model IA ( 191-mm MLL and 48-mm mesh nets).