Publication Date

1-2002

Series Number

131

Publisher

Department of Fisheries, Western Australia

City

Perth, Western Australia

ISBN

0 7309 8459 1

ISSN

1035 - 4549

Abstract

Pilchards have been the primary target of purse seine fisheries along the south and lower west coasts of WA for nearly 15 years. Western Australia’s pilchard population consists of separate west coast and south coast breeding stocks. The adult pilchards in the south coast stock can, in turn, be further divided into three separate assemblages. These adult assemblages correspond to the purse seine fisheries at Albany, Bremer Bay and Esperance. However, while the separateness of the adult assemblages is well established, the status of the juveniles within the context of these adult groups is not understood. For example, the habitats used as nursery areas by pilchards have not been discovered. This lack of knowledge represents a problem for managing the three south coast pilchard fisheries. There is thus a need to determine the relationship between juveniles from the different assemblages of adult pilchards amongst regions of southern WA. In particular, whether juveniles which originate in each region largely remain separate or mix together needs to be determined. Following this, knowledge on the rates of mixing of pre-recruits should be investigated so that the relative contribution from any one region to any other region can be estimated.

This project was designed as a pilot study with the aims of:

1. determining if specific pilchard nursery areas exist or if juveniles are simply spread out along the south coast,

2. undertaking chemical analyses of the otoliths of juvenile pilchards from each fishing zone, including those from samples obtained in previous years, to determine if separate groups of pre-recruits can be identified consistently over several years.

3. determining if large numbers of pre-recruit pilchards can be tagged using chemical dyes to mark otoliths or other bones

4. assessing whether there is a potential to develop a fishery independent index of recruitment.

Since industry infrastructure was a key part of the intended field program to catch juvenile pilchards, the post pilchard-mortality downturn, and subsequent cessation, of commercial pilchard-fishing activity at Albany and Bremer Bay seriously impacted the ability to search for juvenile pilchards in these two regions. Thus, effort was concentrated in the Esperance region, the only to retain a commercial pilchard fishery 2 Fish. Res. Rep. West. Aust. 2001, 131, 1-44 during the project. Juvenile pilchards were successfully caught in research nets but the catch rates were low. Because fishing for juvenile pilchards was significantly more difficult for Fisheries WA staff than anticipated, only one site at Esperance could be sampled on a regular basis. This negated the opportunity to search for juvenile pilchards in different areas and therefore the existence, or not, of specific nursery areas could not be ascertained using direct capture techniques. Considering the high level of effort required for researchers to catch juvenile pilchards, sampling of the commercial catches by Fisheries WA staff continues to provides the best means of assessing recruitment levels each year.

Chemical analysis of the otoliths indicates that in some years juveniles that move in to each of the three regions had lived in similar habitats prior to recruitment but in other years had lived in quite different habitats. The similarity of habitats occupied by juveniles in some years supports a hypothesis of a large, single pool of recruits that supplies each adult assemblage. However, the differences in other years suggests that there can be up to three distinct groups of recruits which have utilized different nursery areas.

Pilchards caught in Esperance using a commercial vessel and held in a cage for several weeks were given a non-toxic chemical dye mixed into a high-energy food. Although the food was readily eaten by the pilchards, analyses in the laboratory could not detect any of the chemical on either the otoliths or other bones. This study was therefore able to show that chemical marking with dyes is not a useful tagging methods for pilchards in WA. Unless other methods of tagging can be developed, the origin(s) of juvenile pilchards within each of the three southern WA fishing zones cannot be resolved with such methods. Techniques which examine DNA may assist with determining the relationships between juveniles from the different zones.

Number of Pages

44

Keywords

Purse seine fisheries, Albany, Pilchards, Sardinops sagax, Pilchard larvae, Leeuwin Current, Pilchard fishery, Western Australia

Disciplines

Aquaculture and Fisheries | Environmental Monitoring | Marine Biology | Natural Resources and Conservation | Natural Resources Management and Policy

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