Common dolphin (Delphinus delphis) bycatch in New Zealand mackerel trawl fisheries, 1995–96 to 2007–08

Common dolphins (Delphius delphis) are the most frequently caught cetacean in New Zealand trawl fisheries. Between the 1995—96 and 2007—08 fishing years, there were 97 common dolphin captures reported by Ministry of Fisheries observers in the mackerel trawl fishery on the west coast of the North Island. These captures were all made by vessels over 90 m in length. Capture events were infrequent, with 1 trawl in every 100 recording common dolphin captures. However, when common dolphins were caught, they were often caught in groups. In 2007—08, 20 common dolphins were caught on 5 of 725 observed tows. There were 9 common dolphins caught on a single trawl.

A statistical model was built that used the observer data to estimate the total common dolphin captures in the mackerel fishery. A two-stage Bayesian hurdle model was used, with a logistic generalised linear model predicting whether any common dolphin captures occurred on a given tow, and a zero-truncated Poisson distribution being used to estimate the number of dolphin captures, given that there was a capture event.

Of the observed capture events in the large vessel mackerel fishery, 54% were during the 9% of observed trawls where the top of the net was less than 30 m below the surface. All the 2007—08 capture events occurred on trawls with a headline depth of 20 m or less. The model found that headline depth was the covariate that best explained the occurrence of common dolphin captures, with the probability of a dolphin capture event on a tow being halved by increasing the headline depth by 17 m. Both the model and the raw data suggest that restricting trawls with shallow headlines would reduce dolphin bycatch. Trawl duration, light condition, and region were all also identified as covariates associated with dolphin bycatch. The model estimated that there was a higher bycatch on longer trawls, trawls hauled between midnight and dawn, and trawls in the region to the north of 30° 18’ S.

There was little effort in the large vessel mackerel fishery before 2000—01; fewer than 600 tows per year. The estimated number of common dolphin captures was also relatively small, with a median of less than 20 captures per year. As the annual effort in this fishery expanded to over 2000 tows by 2002—03, there was an initial increase in the number of common dolphin captures to 184 (95% c.i.: 76 to 394). Since then the number of captures has decreased. In 2007—08 there were an estimated 44 (95% c.i.: 24 to 78) common dolphins killed in the large vessel mackerel fishery. The reasons for the decrease in common dolphin captures since 2002—03 are not known. There are no available estimates of the number of common dolphins living in the region where the mackerel fishery is active, and so there is no basis for assessing the impact of these mortalities on the local common dolphin population.