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Evaluation of egg production as a method of estimating spawning biomass of redbait off the east coast of Tasmania

Neira, FJ, Lyle, JM, Ewing, GP, Keane, JP and Tracey, SR ORCID: 0000-0002-6735-5899 2008 , Evaluation of egg production as a method of estimating spawning biomass of redbait off the east coast of Tasmania.

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During the mid-1980s a major purse-seine fishery for small pelagic fishes, mainly jack mackerel (Trachurus declivis), developed off Tasmania. Catches exceeded 40,000 t in the late 1980s but were prone to large inter-annual fluctuations, due mainly to variation in the availability of surface schooling fish. By 2000 purse-seine fishing had effectively ceased, at which time industry commenced trialling mid-water trawling for small pelagics. Redbait (Emmelichthys nitidus), rather than jack mackerel, comprised the bulk of the trawl catch.

Since the late 1980s, the fishery has been managed using a combination of input and output controls, including a total allowable catch (TAC). The initial TAC was set at 42,000 t, based on the highest purse-seine annual catch. The TAC was decreased to 34,000 t (combined species TAC) for the 2002-03 and subsequent fishing seasons but without any scientific basis. At the same time the fishery had changed from a purse-seine fishery for jack mackerel to a mid-water trawl fishery primarily targeting redbait. Management of the fishery, which now forms part of the Commonwealth Small Pelagic Fishery (SPF), is currently under review but will be based on output controls (quotas).

The primary objective of the present study was to evaluate the suitability of the daily egg production method (DEPM) for redbait. The DEPM is a fishery-independent method currently used worldwide to provide spawning biomass estimates of small pelagic fishes that are serial or batch spawners, i.e. release batches of pelagic eggs into the water column throughout the spawning season. The method assumes that the biomass of a fish population can be estimated from the daily production of eggs over the spawning area, the weight and proportion of females that spawned on a given day, and the average fecundity of each reproductively active female. As such, there are several characteristics relating to the reproductive biology, spawning dynamics and development of eggs that are prerequisites for the method.

Redbait reproductive biology was examined from fish caught off eastern and south-western Tasmania. Redbait are batch spawners with asynchronous oocyte development and indeterminate fecundity, spawning over a discrete 2-3 month period during spring. Size at maturity varied markedly between eastern and south-western Tasmania, with south-west coast fish attaining 50% maturity at sizes nearly 100 mm larger than off the east coast. Correspondingly, ages at maturity also differed by region, with east coast redbait maturing at around 2 years of age compared to 4 years in the south-west. Examination of functional oocyte groupings and histology of post-ovulatory follicles indicated that females spawned once every three days, and that peak spawning occurred before midnight. The relationship between batch fecundity and ovary-free body weight was linear, with fecundity increasing at a rate of 186 oocytes per gram of weight.

Attributes of the reproductive biology and spawning dynamics of redbait indicated that the species was suitable for the application of the DEPM. Adult DEPM input parameters estimated for east coast redbait in 2005 and 2006 were, respectively: sex ratio (R) = 0.30, 0.44; female weight (W) = 71.7, 78.3 g; batch fecundity (F) = 10,894, 11,441; and spawning fraction (S) = 0.32, 0.32.

Reared eggs and field-collected material were employed to describe the development of the pelagic eggs and larvae of redbait. Hydrated oocytes from adults trawled from spawning grounds were fertilized and reared to the yolk-sac larval stage, and the data employed to build a temperature-dependent egg incubation model. Embryogenesis lasted 96, 84 and 54 hours at mean temperatures of 13.1, 14.4 and 16.5oC, respectively, and was divided into seven stages based on extent of epiboly until blastopore closure (stages I-III) and embryo growth (stages IV-VII). Morphological identification of eggs collected during ichthyoplankton surveys was validated using a molecular primer/ probe combination capable of isolating a 120 base-pair segment of the mtDNA d-loop gene region unique to redbait. The probe was tested using real-time polymerase chain reaction (PCR) amplification of DNA, producing an 80-100% agreement across all egg stages. Variability of mean egg ages (y) by temperature (t) and stage (i) was best described by the deterministic stage-to-age model

y = 35.911 e-(0.155t + 0.262i) i (2.436).

The development of this incubation model to assign ages to staged field-caught eggs of redbait represented a significant achievement in terms of its application to estimate spawning biomass using the DEPM.

Spawning habitat of redbait was described from egg, larval and environmental data collected over shelf waters between north-eastern Bass Strait and the lower south-west coast of Tasmania in October 2005 and 2006. Egg data were further analysed to estimate the spawning areas off eastern Tasmania, while daily egg abundance-at-age data were employed to compute mean daily egg production (P0) and instantaneous mortality (Z) estimates. Eggs occurred along the entire area sampled in 2005 (15,650 km2; 38.8-43.5oS). By contrast, 96% of the eggs caught in the much larger area sampled in 2006 (21,351 km2) came from the shelf off eastern Tasmania (40.5-43.5oS) while very few occurred south of 43.5oS along the southern to south-west coasts (145.5-147.7oE). The distribution and abundance of redbait eggs and larvae indicated that spawning takes place mostly along a 5 nm corridor over the shelf break, in average mid-water temperatures of 13.5-14.0oC. This observation was supported by the significantly greater abundances of day-1 eggs at shelf break than at either shoreward or offshore stations. Estimated spawning areas (% of total survey area) were 13,220 km2 (84.5%) in 2005 and 8,695 km2 (40.7%) in 2006.

Mean P0 and Z were computed for two data scenarios by fitting a least squares non-linear regression model (NLS) and a generalised linear model (GLM), with the latter providing a better description of the data. Excluding eggs assigned ages ≤ 4 hours and ≥ 98% of incubation time, GLM-derived P0 (eggs/0.05m2 day-1) was estimated as 4.04 both in 2005 (CV 0.14) and 2006 (CV 0.19), with Z of 0.37 (CV 0.24) and 0.50 (CV 0.34) for 2005 and 2006, respectively. Total egg production per spawning area (eggs x 1012) was 1.26 in 2005 and 1.05 in 2006. Biomass estimates based on the preferred model (GLM with extreme egg cohorts excluded) were 86,990 t (CV 0.37) in 2005 and 50,782 t (CV 0.19) in 2006.

In conjunction with previous ichthyoplankton surveys, our data support the proposition of a discrete eastern spawning stock that splits around southern Tasmania and extends north into southern NSW. Given the geographical extent of this stock and the fact that the entire spawning area was not surveyed in this study, spawning biomass estimates are almost certainly conservative.

This study has conclusively established that redbait is a suitable species for the application of the DEPM, with the results providing a sound scientific basis on which to base harvest levels. Additional work is nevertheless required in various technical areas of biomass estimation. Potential improvements in terms of acquisition of egg and adult reproductive data to augment precision of the DEPM application, including testing of alternative model scenarios, are highlighted. More importantly, however, future surveys will need to cover the entire shelf distribution of redbait to ensure that spawning biomass estimates truly reflect current stock abundance.

Item Type: Report (Project Report)
Authors/Creators:Neira, FJ and Lyle, JM and Ewing, GP and Keane, JP and Tracey, SR
Keywords: Redbait, Emmelichthys nitidus, Daily Egg Production Method (DEPM), spawning cycle, egg and larval development series, genetic identification of fish eggs, plankton survey, Small Pelagic Fishery, biomass assessment
Publisher: Tasmanian Aquaculture and Fisheries Institute
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