Investigating the catchability of the New Zealand rock lobster Jasus edwardsii, with aspects to fisheries ecology

Abstract

Catchability of the New Zealand rock lobster Jasus edwardsii was investigated by studying aspects of fisheries ecology, focussing on the CRA 2 management area in north-eastern New Zealand, and in particular the area around Leigh. Lobster catchability depends on the probability of animals encountering, entering and being retained in traps. Therefore a simple and cost-effective camera system ‘Red Eye’ was developed to analyse and observe behaviour around commercial traps. The Red Eye camera trap did not influence fishing, indicating an accurate portrayal of lobster behaviour. Less than half of the lobsters that approached the trap were caught, with some lobsters not even contacting the trap and peak lobster activity occurring 2 ½ hours after dusk. Catch from trap surveys were measured and sexed, and compared to lobsters measured and sexed on the seabed from diver surveys within the Cape Rodney to Okakari Point Marine Reserve, and in fished areas outside the reserve. Trap surveys were found to be size and sex selective, catching larger lobsters, and had a different sex ratio to the population on the seabed. Trap catch appeared to be slightly correlated with density of lobsters on the seabed. Size at the onset of sexual maturity (SOM) was determined for lobsters by collecting measurements from trap catches and local fisherman catch. Both males and females matured below the current minimum legal size of the CRA 2 fishing area, which is important for managing J. edwardsii, allowing them to breed before they become available for the fishery. Generalised linear modelling of dive survey data was used at the transect level to determine fine scale habitat that accounted for lobster abundance. General and legal lobster abundance was positively correlated with shelter availability, Ecklonia radiata, marine reserve status and the interaction of E. radiata and depth. Cohabitation data of nesting lobsters was also analysed and patterns over time were investigated. These findings have significant implications for fisheries management as information around catchability and fisheries ecology of J. edwardsii is necessary for stock assessments, ensuring the fishery remains sustainable and productive.