Spatially explicit benthic impact assessments for bottom trawling in New Zealand

The spatial extent of bottom contact by mobile fishing gear by New Zealand’s inshore and deepwater fisheries has been mapped in detail. However, understanding the effect of bottom fishing on benthic communities over this area is far more complex and is dependent on the actual impact of fishing to a given taxon or community. The aim of this project was to apply, assess, and further develop methods to quantify impacts of fishing on benthic taxa or communities within New Zealand’s Territorial Sea (TS) and Exclusive Economic Zone (EEZ).

A variety of methods for spatially explicit benthic impact assessment have been developed, and implemented, to quantify the likely bottom impacts of fishing. These methods combine information about parameters including: historical fishing effort; gear type; gear deployment; substratum/habitat type; vulnerability of taxa or communities; and the distributions of benthic taxa, communities, or ecosystems. In data-poor situations, particularly in the absence of reliable data or models for the distribution of benthic taxa or communities, proxy data have been used in these impact assessments.

This project used two published methods to assess benthic impact at the scale of the TS and EEZ and explored the development and application of two additional methods at the scale of the Chatham Rise. Before these methods could be applied, the types of mobile fishing gear used in the New Zealand inshore (since 2007/8) and deepwater (since 1989/90) fisheries were characterised and categorised, and the spatial and temporal extent of bottom contact was determined for the different fishing gear configurations.

The first benthic impact assessment method (here referred to as the MSRP method using the first initials of its authors) combines information on gear categories, expert opinion on the vulnerability of three benthic functional groups to trawl gear, and the bottom contact footprint of trawl fishing. The second method, relative benthic status (RBS), relied on the determination of the swept area ratio of trawling in the region; RBS was calculated for different vulnerable marine ecosystem (VME) indicator taxa. These methods, as applied here, do not consider spatial distributions of any specific benthic taxa, but such distributional layers can be incorporated in a subsequent step—as long as the taxon fits within one of the defined functional groups/VME indicator taxa used by these methods. The availability of the distributional layers, along with estimates of vulnerability parameters, for other benthic taxa or communities is critical for the wider application of this approach.

The third method used in the present study was a modification of the RBS approach which used trawl and benthic bycatch data from the Chatham Rise in simulations to explore the underlying assumptions of RBS, as well as statistical models to directly estimate depletion and recovery parameters for four benthic taxa from these local data. The results of this work were encouraging, but further development is required before this enhanced approach can be implemented more widely and with confidence. The fourth method updated a previous application of a spatial-temporal modelling approach which assessed the impact to a soft-sediment habitat indicator taxon on the Chatham Rise and extended its application to a hard substratum indicator taxon. This approach is also reliant on estimates of benthic taxa depletion, and further development of this approach is required to improve its utility.

While further development of the second two methods is required, outputs from the application of the complementary MSRP and RBS methods can now be used to inform marine spatial planning processes in New Zealand and can support inshore and deepwater Fisheries Management in meeting objectives in their respective management plans.