Analyses of the composite yield per recruit model CYPR14 for inferring plausible fishing mortality targets of fish in the tropics

Abstract

Stocks' yield and size per recruit are widely used to provide fisheries management guidance. This study provides details for analyzing the composite (i.e. age-aggregated or stage-structured) yield per recruit (CYPR) model CYPR14, and proposes CYPR14 as a management tool for tropical fisheries. The fishing mortality rates maximizing CYPR (FCYPR) and associated with the marginal increase in CYPR (F0.1) and a target composite spawning potential ratio (CSPR; F35%CSPR or F40%CSPR) were suggested as candidate fishing mortality targets, provided assessments employ the delay-differential model underlying CYPR14. Using Monte Carlo (MC) simulations relying on growth parameters and natural mortality of Lake Tanganyika's Lates stappersii and Lake Victoria's Lates niloticus, CYPR14 analyses involving maximum survivorship or declining survivorship were carried out to show how FCYPR, F0.1, F35%CSPR, and F40%CSPR could be generated, given an age of knife-edge recruitment (r). Baseline MC employed r = 1 year and yielded mean annual rates of FCYPR = 0.52, F0.1 = 0.33, and F35%CSPR = 0.51 for L. stappersii and FCYPR = 0.23, F0.1 = 0.14, and F40%CSPR = 0.16 for L. niloticus. CYPR14 with maximum survivorship produced CYPR isopleths such that the CYPR maximized at an infinite r and finite, higher F. For CYPR14 involving a declining survivorship, the CYPR declined with increased r and maximized with innermost closed-loop contours at lower F and an optimal age. The CSPR isopleths from both types of CYPR14 analyses were first concave down, and the optimal age served as their inflection point. In terms of benchmarks based on the maximum sustainable yield and of proxies thereof, CYPR14 should be for its underlying delay-differential model what the age-structured pool models are for age-structured assessment models.