EXECUTIVE SUMMARY
Hart, A.M.; Surveys of pre-recruits of Jasus edwardsii in CRA 8 using modified commercial pots – a pilot study.
A pilot study was undertaken in the CRA 8 Jasus edwardsii fishery to investigate the feasibility of measuring pre-recruit abundance and size using modified commercial pots.
Five rock lobster vessels set experimental pots from August to November 2001 at 14 sites between mid-Fiordland and eastern Stewart Island . At each site, pots were set in pairs of one normal commercial pot (N) and one without escape gap (WE) pot, and lifted between 5 (Site 10) and 29 (Site 3) times.
All lobsters were counted; their tail width measured and sex status identified, and the data recorded on data sheets adapted from the logbook program. The catches of WE and N pots were compared statistically using 10 abundance and 10 size indicators, which included total number caught, overall average size, number of pre-recruits (under 54mm Tail Width [TW]), average size of pre-recruits and average size of large lobsters (over 60mm TW).
A total of 6592 lobsters between 28 and 90mm TW were sampled, measured and sexed. Of these 4528 (69%) were caught in pots without escape gaps (WE), and 2064 (31%) were caught in normal pots (N). The difference in numbers caught between the two pot types was due to the pre-recruit portion of the population, which were caught in far greater quantities by WE pots. WE pots sampled animals as small as 28mm TW and clear differences in size-frequency between sites were evident.
There was no statistical difference between pot types in estimating parameters of the post-recruited population; the WE pots caught similar numbers and sizes of post-recruits, berried females and large lobsters (greater than 60mm TW). The difference in average size of large lobsters measured by the two pot types was 0.4 mm. Post-hoc power analysis revealed that all tests were sufficiently powerful to detect small differences. For example, minimal detectable difference (between WE and N pots) in numbers of post recruit (above 54mm TW) lobsters was 1.1 lobsters per pot (19% of a mean of 5.7); the minimal detectable difference for average size of post-recruits was 1 mm (2% of a mean of 57.9 mm).
Commercial pots with closed escape gaps provide substantial potential for monitoring the abundance and size of pre-recruit Jasus edwardsii in CRA 8. Optimal sampling analysis determined that a pre-recruit monitoring program with 120 sites and 3 potlifts per site, would achieve desired levels of precision for about the same cost (effort) of the pilot study. A site allocation regime is necessary to ensure these sites are representative of the population. If a pre-recruit index of abundance is required for management of the CRA 8 fishery, a monitoring program using WE pots could be implemented, with due consideration to quality control issues as discussed within the report. Further tests of the sampling efficiency of WE pots would be advantageous, and detailed recommendations are provided. |
