Common dolphin captures in 2008–09

Estimates of common dolphin bycatch in the the jack mackerel trawl fishery over a 13-year period were recently published by Dragonfly in a report based on data from 1995 to 2009. Common dolphins (Delphinus delphis) are the most frequently observed cetacean caught in New Zealand trawl fisheries.

Between the 1995–96 and 2008–09 fishing years, 108 common dolphin captures were reported by Ministry of Fisheries observers in the mackerel trawl fishery on the west coast of the North Island. Dolphin captures were then estimated across all fishing effort, based on data from observers.

“We estimated fewer than 5 dolphins a year were captured between 1995–96 and 1998–99, but as effort in the fishery increased there was a large increase in the estimated number of mortalities, which peaked at 182 dolphins in 2002–03. Since then, captures have decreased and in 2008–09, the model estimated that 25 dolphins were killed”, said Finlay.

The statistical modelling used the observer and associated fishing effort data to estimate dolphin captures across the bulk of the fishery. A two-stage Bayesian hurdle model was used to estimate the probability of a dolphin capture event occurring, and if a capture event did occur, the number of dolphins killed.

The model also explored several factors that might contribute to dolphin captures. The strongest relationship was between dolphin captures and the headline depth (depth of the top of the trawl net).

“We found that dolphins were more likely to be caught when the headline depth was less than 30 m, than when it was deeper. Keeping the top of the net below 30 m depth would reduce the number of captures.”

Common dolphin are the most numerous dolphin species in coastal waters around New Zealand and sometimes form schools of up to several thousand individuals. Dolphin captures are reasonably infrequent, but because dolphins feed in groups, capture events typically involve 2 to 3 dolphins. A maximum of 9 common dolphins were once caught in a single tow.

“Although nearly 80% most trawl effort in the area is targeting inshore fish species, there is still a big gap in our knowledge about how many dolphins are being captured in the inshore trawl fisheries. Over the period covered by this work, observer coverage in inshore fisheries was less than 0.5% per fishing year. This wasn’t enough to allow estimation of captures in this fishery.”

The research was carried out for the Ministry of Fisheries and is published in the New Zealand Aquatic Environment and Biodiversity series, No 63.

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