Trawl surveys of the Hauraki Gulf and Bay of Plenty in 2019 and 2020 to estimate the abundance of juvenile snapper.

Abstract

This report presents the results of inshore trawl surveys of the Hauraki Gulf and Bay of Plenty using the R.V. Kaharoa. It has been 19 years (Hauraki Gulf) and 20 years (Bay of Plenty) since these trawl surveys were last conducted. Previous surveys were conducted between 1984 and 2000. The recent surveys took place between 12 and 30 November 2019 (Hauraki Gulf) and 12 February and 1 March 2020 (Bay of Plenty) and used a stratified two-phase design optimised for pre-recruit snapper. The survey design involved removing deeper strata from the historic survey stratification to better target pre-recruit snapper. A total of 39 phase one and 5 phase two stations were completed for the Hauraki Gulf, and 42 phase one and 10 phase two stations were successfully completed in the Bay of Plenty. Phase two stations were conducted solely to reduce the coefficient of variation (CV) for pre-recruit snapper. In the Hauraki Gulf, the estimated year class strength (YCS) of 1+ and 2+ snapper (the 2018 and 2017 year classes, respectively) was 5.4 (CV = 20.0%) and 7.4 (CV = 11.0%) million fish, respectively. These estimates are both well above the average YCS estimates for the entire Hauraki Gulf survey series (average YCS is 3.8 and 3.6 million for 1+ and 2+ snapper, respectively, in 12 previous surveys from 1984 to 2000), but well below the YCS predictions based on a previously established temperature-recruitment relationship. In the Bay of Plenty, the estimated YCS of 1+ and 2+ snapper (the 2019 and 2018 year classes, respectively) was 0.926 (CV = 27.4%) and 0.453 (CV = 23.3%) million fish, respectively. For 1+ snapper, the recent estimate is well above the average YCS for the Bay of Plenty survey series (the average 1+ YCS was 0.307 million fish). For 2+ snapper this estimate is very similar to the average YCS for the Bay of Plenty survey series (the average 2+ YCS since 1990 is 0.465 million fish). Future surveys in the Bay of Plenty should pay close attention to the spatial distribution of pre-recruit snapper, which was constrained to the eastern Bay of Plenty in the 2020 survey. The recommencement of the Hauraki Gulf and Bay of Plenty trawl surveys series may also provide potential to monitor, with reasonable levels of precision, the relative abundance of year classes over 2 years (i.e., including recruited snapper). In the Hauraki Gulf, the estimated snapper biomass was 20 618 t (CV = 13.6%), whereas in the Bay of Plenty it was 2824 t (CV = 8.9%), representing increases of 139% and 87% since the last survey in each area, respectively. Other species that the Hauraki Gulf and Bay of Plenty surveys may have some potential to monitor include: red gurnard (Hauraki Gulf: 162 t, CV = 28.3%; Bay of Plenty: 181 t, CV = 14.6%), John dory (Hauraki Gulf: 188 t, CV = 14.7%; Bay of Plenty: 84 t, CV = 23.3%), and potentially leatherjacket (Hauraki Gulf: 338 t, CV = 30%; Bay of Plenty: 149 t, CV = 55.7%) and trevally (Hauraki Gulf: 107 t, CV = 34.7%; Bay of Plenty: 74 t, CV = 8.2%). For the Hauraki Gulf, the biomass of all these species was higher than that estimated by the previous survey. Conversely, in the Bay of Plenty, red gurnard, John dory, and trevally had all decreased (and leatherjacket had increased) relative to the previous survey estimates.