Currently the size of the Foveaux Strait oyster population is influenced more by disease mortality (specifically, the parasite Bonamia exitiosa) than by fishing.

Here we update the previous oyster stock assessment from 2017, by adding the most recent data from the catch history, catch-per-unit-effort, commercial catch sampling, and estimates of population sizes for three difference sizes of oysters.

The first innovation we introduce is to model disease in more detail, by keeping track of the numbers of diseased oysters, newly deceased oysters and older deceased oysters.

The second innovation is to simulate disease mortality five years into the future, based on the disease events seen in the past.

It is estimated that the current (2023) population size for the base case is approximately 940 million oysters, which is 32.6% of the virgin biomass (maximum population size).

It appears from the data that we entered another disease mortality event in 2023.

The abundance in 2028 is predicted to be around 31% of the virgin biomass for the base case, with the alternative models giving similar abundances.

An acoustic survey of orange roughy on the southwest Challenger Plateau (including Westpac Bank) was carried out from 1 to 15 July 2023 by the National Institute of Water and Atmospheric Research Limited (NIWA) onboard research vessel (RV) Tangaroa.

Two acoustic instruments used were: the acoustic-optical system; and the dual-frequency towed body. These instruments are towed at depth and operate dual frequencies (38 and 120 kHz), both of which are needed to differentiate orange roughy from other species.

Acoustic marks of orange roughy were validated using the NIWA orange roughy bottom trawl. Trawling informs the species composition of the spawning aggregations and provides biological data.

RV Tangaroa was used to complete searches for orange roughy around all known areas or features where aggregations have been known to occur on the Challenger Plateau. The searches detected aggregations of orange roughy at Megabrick and Twin Tits (the Pinnacles seamount complex), and Volcano on the Westpac Bank.

Despite extensive searches carried out on four separate occasions during the survey, no aggregations of orange roughy were found on the Challenger Flats area.

Acoustic surveys (or snapshots) on aggregations of orange roughy carried out on Megabrick produced biomass estimates of 1380 t and 290 t, respectively.

Acoustic surveys on aggregations of orange roughy carried out on Volcano produced biomass estimates between 6461 t and 10 255 t.

Too few fish were caught to determine if spawning of orange roughy was underway during the 2023 acoustic survey.