Applicability of Egg Surveys for Spawning-Stock Biomass Estimation of Snapper, Orange Roughy, and Hoki in New Zealand

In New Zealand, large and valuable fisheries are currently prosecuted on spawning aggregations of snapper (Sparus auratus), orange roughy (Hoplostethus atlanticus) and hoki (Macruronus novaezelandiae). This paper designates egg survey techniques appropriate for spawning stock biomass estimation for these fisheries. Snapper are multiple spawners with indeterminate annual fecundity. The stock has a well defined, daily rhythm of ovulation and spawning, with high spawning frequency. Aspects of planktonic egg production are reasonably well understood. Thus, the daily egg production method appears viable as a method of biomass estimation. Orange roughy have determinate fecundity, and because temporal and spatial distributions of spawning are well known from trawl data on adult ovarian state, it is likely that the daily fecundity reduction method would be useful for biomass estimation. The annual egg production method would also be useful for orange roughy fisheries where the spatial distribution of spawning is not large. However, use of either method must await description of the embryology and development rates and better description of the vertical and horizontal distribution of the planktonic eggs. Hoki are multiple spawners and probably have determinate fecundity. The annual egg production method would be required for absolute spawning biomass estimation because spawners migrate into the spawning area, spawn, and leave over a period shorter than the spawning season of the stock. It could be used most efficiently in conjunction with acoustics, and, if the spawning fraction can be identified, the daily egg production method, to calibrate these other methods with respect to absolute spawning biomass. Adaptations in spawning strategy of these three fishes from different environments are discussed. The differences seen in fecundity and spawning-season length may be explained as adaptations to maximize survival of larvae, but may also reflect constraints on fecundity and spawning-season length due to egg size and spawning-behaviour requirements.