Common dolphin (Delphinus delphis) bycatch in New Zealand mackerel trawl fisheries, 1995–96 to 2008–09.

Common dolphins (Delphinus delphis) are the most frequently observed caught cetacean in New Zealand trawl fisheries. Between the 1995—96 and 2008—09 fishing years, there were 108 common dolphin captures reported by Ministry of Fisheries observers in the mackerel trawl fishery on the west coast of the North Island. All captures were by vessels over 90 m length. The pattern of captures was similar across fishing years, with a general increase in fishing effort and a concomitant increase in common dolphin captures over the 14-year period. In the most recent fishing year, 2008—09, fishing effort was comparatively lower than in preceding years, and 11 common dolphins were caught in 4 of 510 tows. Capture events frequently involved the capture of more than one individual, with a maximum of 9 common dolphins observed caught in a single tow.

Based on observer data, a statistical model was built that estimated the total number of common dolphin captures in the large vessel 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 to estimate the number of dolphin captures, given that there was a capture event. The model was also used to explore which covariates were related to common dolphin captures.

There was little effort in the large vessel mackerel fishery before 2001—02, with fewer than 1000 tows per year. The estimated number of common dolphin captures was also relatively small, with a median of less than 12 captures annually. As the effort in this fishery expanded to over 2000 tows by 2002—03, there was an initial increase in the estimated number of common dolphins caught per year to 182 (95% c.i.: 72 to 389). Since then, the number of captures has decreased, although fishing effort has remained relatively high. In 2008—09, there were an estimated 25 (95% c.i.: 13 to 52) common dolphins killed in the large vessel mackerel fishery. The reasons for the decrease in common dolphin captures since 2002—03 are unknown. Lack of dolphin abundance data prevents assessment of the impact of these mortalities on the common dolphin population in the mackerel fishing area.

The model found that headline depth (distance of the headline below the surface) was the covariate that best explained the occurrence of common dolphin captures. Both the model and observer data suggest that restricting trawls with shallow headlines would reduce dolphin bycatch. Of the observed capture events, 50% were during the 10% of observed trawls where headline depth was less than 30 m below the surface. All capture events in 2008—09 occurred on trawls with a headline depth of 20 m or less. Increasing the headline depth by about 13 m would halve the probability of a dolphin capture event on a tow. Light condition, trawl duration, and sub-area were all also identified as covariates associated with dolphin bycatch. The model estimated that there was a higher bycatch on trawls hauled between midnight and dawn, on longer trawls, and on trawls in the northern sub-area of the fishing region (above 30° 18’ S).

In addition to the large vessel mackerel fishery, there have been observed common dolphins captured in inshore trawl fisheries. Although there are large fisheries targeting inshore fisheries (e.g., 78% of all trawls on the west coast of the North Island in the 2008—09 fishing year), observer coverage has been limited to 0—0.5% per fishing year. Because of the low observer coverage, it is not possible to estimate the number of common dolphins caught in inshore fisheries.