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Population monitoring programmes are used to gain knowledge about system functioning or about the response of a system's state to management measures. The current study provided an assessment of comprehensive monitoring data of intertidal bivalve populations in northern New Zealand. In this region, a number of bivalve species are targeted in recreational and customary fisheries, including cockles (Austrovenus stutchburyi) that are one of the main target species. To monitor the sustainability of cockle populations, regular population surveys have been conducted since the 1990s at different northern New Zealand sites, although the sampling frequency has varied greatly across sites. Furthermore, a general lack of fishing data for the target populations prevents a systematic assessment of management measures and population trends within a fishery assessment framework. In view of these shortcomings, the current study provided a systematic phenomenological assessment of the survey data to make general recommendations about the effectiveness of management and for guiding prioritisation and design for future surveys. The assessment was based on a Bayesian state-space model that simultaneously analysed monitoring data from 21 incomplete survey time-series between 1999-2000 and 2014-15. The model showed that the predictability of estimated large-size cockle densities was highly site-specific. Across all sites, the predictability of density trends could be used to guide the prioritisation of sites to be included in the survey, or to adjust the sampling frequency at individual sites. Based on the current analysis, existing management measures in the form of fishery closures at some of the sites appear to be effective in allowing recovery of depleted cockle populations. Only those sites that had management measures in place showed an increase in large cockle densities. In the absence of fishing data, adaptive management and targeted intervention could be used to further assess the efficacy of management measures.
This study provided a first comprehensive assessment of cockle surveys in New Zealand's northern North Island. It has been presented to and reviewed by the Shellfish Working Group convened by the Ministry for Primary Industries (MPI), as well as by MPI staff.