DEPM-based spawning biomass of Emmelichthys nitidus (Emmelichthyidae) to underpin a developing mid-water trawl fishery in south-eastern Australia

Abstract

Spawning biomass of redbait, Emmelichthys nitidus (Emmelichthtyidae), was estimated using the daily egg production method (DEPM) based on egg and adult surveys conducted simultaneously off eastern Tasmania during October 2005 and 2006. Concurrent studies had confirmed that this mid-water, schooling species met all necessary requirements for DEPM-based biomass estimation, including (a) asynchronous oocyte development with release of pelagic eggs in batches; (b) spawned eggs could be assigned ages using a temperature-dependent incubation model; and (c) egg abundances follow the typical exponential decay model. Main spawning areas were identified between north-eastern Bass Strait (38.8°S) and south of the Tasman Peninsula (43.5°S) in 2005 (13220 km 2), and between Cape Barren Is. (40.5°S) and the same southern boundary in 2006 (8695 km 2). Daily egg production ( P 0) was estimated by applying two statistical estimation methods to the egg abundance-at-age data, namely the traditional least squares non-linear regression (NLS) and a generalized linear model (GLM). Results indicated that the latter technique provided a better fit, resulting in improved CVs and AIC statistics over the NLS. The GLM-derived average P 0 was estimated at 4.04 eggs/0.05 m 2/day both in 2005 and 2006. Spawning biomass (CV) was ∼87000 t (0.37) in 2005 and ∼50800 t (0.21) in 2006, with the lower 2006 biomass largely due to a smaller spawning area and higher sex ratio. Estimates are likely to be negatively biased since spawning of E. nitidus probably extends as far north as southern New South Wales (35.0°S). In the absence of comparable studies on other emmelichthyids, P 0 and instantaneous egg mortality estimates are compared to those of clupeoid species previously subjected to DEPM, and the method discussed in terms of its suitability to this species in support of a developing mid-water trawl fishery in south-eastern Australia.