posted on October 18, 1996 13:22
Joseph DeAlteris and Ralf Riedel
University of Rhode Island
Department of Fisheries, Animal and Veterinary Science
Kingston, R. I. 02881, USA
Source - Journal of Northwest Atlantic Fishery Science, Volume 19: 73-82
A discrete time model was developed to evaluate yield and spawning stock biomass-per-recruit and yield-per-unit-effort for a cohort of an idealized groundfish. This fish was characterized as relatively long lived (M = 0.2), slow growing (K = 0.2), with maximum length and weight of 100 cm and 10 kg, respectively, and 50% maturity at a relatively early age of 3 years. The size selection characteristics of trawls and hooks were described by a logistic cumulative distribution function (LCDF) with a range of L50 and steepness values. The size selection characteristics of gillnets and traps were described by a scaled normal probability density function (NPDF) with a range of values for Lopt and the standard deviation.
Analysis of isopleth diagrams for yield-per-recruit (YPR) and spawning stock biomass-per-recruit (SPR) for both types of selection functions indicated that YPR is maximized when harvesting is directed on a fish length slightly larger than that at which biomass for the cohort of the unfished population is maximized, and at fishing mortality rates of 2 and greater. Under these harvesting conditions, SPR was between 24 and 36% of the unfished condition. At these levels of fishing mortality and L50, the steepness of the LCDF does not affect the cohort YPR, but does significantly affect the SPR. Likewise, at these levels of fishing mortality and Lopt, the standard deviation of the NPDF also does not affect the cohort YPR, but does significantly affect the SPR. Thus, a sharper selection process provides a greater SPR available for production of future cohorts.
In contrast, yield-per-recruit-per-unit-effort (YPRPUE) is maximized at fishing mortality values of approximately 0.5, when the age at entry or length of susceptibility to fishing gear is set at the age or length of maximum biomass for the unfished cohort.
These results present a dilemma for the fishery resource manager: maximize cohort YPR at fishing mortality values of 2 and greater, with a minimum 65% reduction in YPRPUE, or maximize YPRPUE with a 25% reduction in YPR. However, with compromise, 85% of the maximum YPR can be realized with only a 20% reduction of YPRPUE at a fishing mortality level of 0.75.
Language - English
Publisher - Northwest Atlantic Fisheries Organization (NAFO), Dartmouth, N.S., Canada
Publication Date - September 1996
Publication Type - Journal Article