Assessment of the risk to New Zealand marine mammals from commercial fisheries

This study provides an assessment of the impact of fishing-related fatalities on the populations of 35 marine mammal (sub)species that inhabit New Zealand waters. The assessment included mortalities caused by trawl, longline, set-net and purse-seine fisheries within New Zealand’s Exclusive Economic Zone (EEZ). The risk assessment was an implementation of the Spatially Explicit Fisheries Risk Assessment (SEFRA) method. Risk was defined as the ratio of Annual Potential Fatalities (APF; an estimate of the number of marine mammals killed in fisheries each year) to the Population Sustainability Threshold (PST; a measure of the population productivity). A risk index higher than one indicates that fisheries mortalities are at a level that may prevent the population increasing to, or remaining above, half the carrying capacity in the long term.

In New Zealand, a proportion of commercial fishing is monitored by independent observers. Marine mammal captures are recorded by observers when they are on board fishing vessels. The vulnerability of the marine mammals to capture was estimated from the relationship between observed captures and the overlap of marine mammal species and observed fishing effort. From this vulnerability, the total annual potential fatalities were estimated in all fishing effort. The annual potential fatalities include an estimate of fatalities that are not recorded by observers, and they also allow for the post-release survival of some live-captured animals. Estimates of annual fishing-related mortalities were derived for averaged fishing effort over the three-year period from 2012–13 to 2014–15. The scope of this assessment did not include fishing methods without routine observer coverage, even where marine mammal interactions are known to occur. In particular, the entanglement of marine mammals in the rock lobster (Jasus edwardsii) pot fishery was not included in this study.

Estimation of the PST required information on the maximum growth rate and the total population size for each of the 35 marine mammal taxa included in this study. In the absence of reliable data, demographic and distribution information was estimated via a Delphi survey of marine mammal researchers. The Delphi survey provided participants with a summary of available data, and requested information of the spatial distribution, population size, and maximum population growth of marine mammal taxa. For Hector’s dolphin, population data were also used from a recent survey of the South Island; for Māui dolphin distribution information was used from a recent risk assessment.

Of the 35 taxa, common dolphin was the species with the highest estimated risk, with a mean risk of 1.6 (95% c.i: 0.43 to 4.47). Common dolphin were estimated to be caught in trawl and set-net fisheries, and many of the estimated captures were primarily in poorly-observed small-vessel trawl fisheries in the Taranaki region. Killer whale was the other species with a mean risk higher than one (1.28; 95% c.i.: 0.00 to 7.55). No captures of killer whale have been observed, and the considerable uncertainty in the risk ratio was partly due to overlap between killer whale and poorly-observed coastal fisheries. The PST for killer whale was estimated to be 1.5 (95% c.i.: 0.5 to 3.6) annual fatalities. This value indicates that only a small number of annual fisheries-related fatalities will impact the New Zealand population. The risk ratio for Hector’s dolphin was entirely below one; however, overlap between Hector’s dolphin and set-net fisheries was almost entirely on the East Coast South Island. The risk to the East Coast South Island sub-population from set-net fisheries was estimated as 0.59 (95% c.i.: 0.21 to 1.33)—there was a 9.4% probability that the fatalities from set-net fishing on the East Coast South Island exceed the PST of this sub-population. The mean risk to Māui dolphin was also less than one (0.47; 95% c.i: 0.00 to 1.33), with the credible interval extending above one. Overlap between Māui dolphin and set-net fisheries was primarily in West Coast North Island harbours, and with set-net fisheries close to New Plymouth. For all other marine mammal species, the median risk was below 0.4. In many cases, the distribution was skewed, with an upper 95% credible interval that extended above one. For almost all whales, including beaked whales, the mean annual potential fatalities were less than one; with the exception of humpback whale that had a mean of annual potential fatalities of 1.4. Observer data are not sufficient for constraining Ministry for Primary Industries such low numbers of captures, and consequently the uncertainty in the estimates was high, with many risk ratios for whales and beaked whales having a coefficient of variation over four.

This risk assessment is the first time that a comprehensive analysis of marine mammal bycatch and its population impact has been attempted for New Zealand fisheries. The assessment highlights the need for improved observer coverage in poorly-observed inshore fisheries, especially set-net and inshore trawl fisheries. For most species, the assessment relied on expert judgement to derive distributions for marine mammals. A quantitative analysis of the distribution of New Zealand marine mammals would help improve the estimation of fisheries-related fatalities.