Physical fluxes, pelagic ecosystem structure, and larval fish survival in Hauraki Gulf, New Zealand

Abstract

The ecosystem supporting snapper (Pagrus auratus) larvae was studied during three spring–summer (November–January 1985–1988) spawning seasons in Hauraki Gulf, New Zealand. Upwelling-favourable winds caused more incursion of shelf water into the Gulf in 1985–1986 and 1986–1987 than in 1987–1988, but in the first two seasons, the winds were relatively weak. Stronger winds in 1987–1988 drove greater vertical diffusivity and correlated with greater mixed-layer primary biomass and productivity. Effects of vertical mixing appeared to dominate horizontal incursion of upwelled shelf water in supporting upper water column productivity. The more productive 1987–1988 season had greater abundances of nauplii, copepodites, adult copepods, cladocerans, chaetognaths, hydromedusae, decapod larvae, and numerous larval fish taxa (including snapper). There was much higher survival of snapper between the late-stage egg and post-first-feeding larval stages in 1987–1988, which correlated spatially and temporally with high larval prey densities. Neither higher egg production, fewer predators, nor less horizontal advection accounted for these strong larval snapper cohorts. We hypothesize that larval competence improved within the superior larval feeding environment, reducing predatory losses. The ecosystem response to wind mixing may partially explain the correlation of sea temperatures with recruitment, previously observed for the Hauraki Gulf snapper stock.