Common dolphin captures in the jack mackerel fishery, 2006-07

Common dolphins

Common dolphins (Delphinus delphis) are the most numerous dolphin species in coastal waters around New Zealand, sometimes forming schools of up to several thousand individuals. However, precise population numbers are unknown. The research presented here estimated the number of common dolphins that are caught in New Zealand fisheries. The capture of other cetacean species was not covered by this research.

Jack mackerel fishery

Dolphin bycatch

In New Zealand waters, common dolphins are most frequently caught in the jack mackerel (Trachurus declivis, T. murphyi, and Trachurus novaezelandiae) trawl fishery, on the west coast of the north island of New Zealand. The photograph shows a common dolphin caught on a jack mackerel trawl (photograph by permission of the Department of Conservation).

Observed dolphin captures

The Ministry of Fisheries observer programme collects data on mammal and seabird captures, including dolphins. Observers typically covered 23 to 39% of the fleet during the 12 year period for which we have data. During the 2006-07 fishing year they observed 11 dolphin captures. These capture events occurred on 5 trawls. Dolphin captures are reasonably infrequent, but when they do occur, typically more than one dolphin is caught at a time. Dolphins feed in groups, and so are often caught in groups.

Estimated total dolphin captures

A statistical model used the observer data and associated fishing effort data to estimate dolphin captures across the bulk of the jack mackerel trawl fishery. A two-stage Bayesian hurdle model was used to estimate, firstly, the probability of a dolphin capture event occurring, and secondly, if a capture event did occur, the number of dolphins killed. The model estimated that there were fewer than 5 dolphin captures a year between 1995-96 and 1998-99. As effort in the fishery increased there was initially a large increase in the estimated number of dolphin mortalities, which peaked at 174 dolphins in 2002-03. Since then the number of dolphins caught each year has decreased. In 2006-07 the model estimated that 52 dolphins were killed in the west coast North Island jack mackerel trawl fishery.

Contributing factors

The model explored several factors that might contribute to dolphin captures. These factors included how long the trawl net is towed for, the time of day and time of year the trawling is carried out, and the depth of the trawl net. The strongest relationship was between dolphin captures and the depth of the top of the trawl net (headline depth). Dolphins were more likely to be caught when the headline depth was less than 30m, than when it was deeper.

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