Assessing evacuation rates and spawning abundance of marine fishes using coupled telemetric and acoustic surveys

Abstract

Assessing the spawning abundance of marine fishes is difficult if spawning periods exceed the residency of individual fish on the spawning grounds. For Atlantic cod (Gadus morhua), which has a protracted spawning period, we use biotelemetric surveys to estimate the rate at which individual fish vacate the spawning ground and develop a method to adjust multiple acoustic-survey results to account for spawner turnover. Two acoustic surveys conducted one month apart (May and June 1998) on a cod-spawning ground in Placentia Bay, Newfoundland, yielded abundance estimates of 220 000 and 210 000 fish of mean length 63 cm. Rates of evacuation from the spawning ground, observed over two separate spawning seasons, were modelled as logistic decay functions with good fit (r 2 =0.96 in 1998; r 2 =0.88 in 2000). Our method estimated that only 8.8% of the fish counted during the second survey were present during the first, and that between 400 976 and 420 842 fish were actually present over the full spawning season. Coupled telemetric and acoustic surveys could be used to estimate spawning abundance in many marine fishes.