Examining the ecological complexities of blackfoot paua demography and habitat requirements in the scope of marine reserve protection

Abstract

A critical question for ecologists and fisheries managers is what drives the demographic processes that dictate the abundance and size structure of ecologically and commercially important species. Marine Reserves (MRs) provide an opportunity to examine species in the absence of human disturbance (i.e. no fishing) and to investigate how habitat type, quantity and condition contribute to yield large individuals and dense aggregations that are typical of a more natural state. However, an improved understanding of the efficacy of marine reserves requires a robust examination of habitats inside and outside reserves to distinguish any reserve effect from a potential confounding habitat effect. Abalone are a valuable nearshore fishery in many parts of the world and many stocks have been overexploited to the point of collapse. Countries striving to rebuild their abalone stocks are utilizing MRs to support viable populations and focusing on habitat requirements that produce large aggregations and individuals. The abalone commonly referred to as the blackfoot paua (Haliotis iris) is a culturally and ecologically important New Zealand (NZ) species and is the focus of customary, recreational and commercial fisheries. However, the demography and growth rates of paua populations are highly variable, with pockets of “stunted” populations occurring throughout NZ. Density-dependent processes, differential juvenile success, variable habitat quality and fishing pressure have all been suggested to influence the fitness of individuals and the demography of paua populations.

My research utilizes MRs to control for fishing activity and thereby to investigate ecological patterns and the effects of habitat on paua abundance and size variability. The main objectives of this thesis were to quantify the response of paua to MR status, distinguish habitat effect from a reserve effect and understand the contribution of habitat variables on demography and growth. Research was conducted within and surrounding five MRs in central NZ. The habitats in and outside MRs were not significantly different in physical and biogenic characteristics, but paua occurred in significantly greater densities and were significantly larger within four MRs compared with outside, illustrating that marine reserves do afford protection for paua. Paua within MRs were significantly more dense and larger in areas of relatively higher wave exposure and dense macroalgal cover.

Despite protection, paua were found to be undersized or “stunted” at Long Island and Horoirangi MRs. I conducted surveys to evaluate the effect of density and the contribution of habitat variables on paua size at two spatial scales across environmental gradients. To further test the hypothesis that habitat effects growth a 12 month translocation experiment was conducted at Long Island MR. The surveys revealed that environmental gradients exist at small and large scales and explained how paua size varied along these gradients. The habitat variables which supported larger size individuals were consistent across both locations, where paua were significantly larger in areas that were exposed with high algal cover than those at sheltered areas with low algal cover. This result was further confirmed by the translocation experiment which revealed that paua translocated from a stunted environment to a normal environment grew significantly more than conspecifics placed at the stunted environment.

To further explore the response of paua to protection and see if patterns were consistent across bioregions in areas with “normal” size paua I conducted research at the Taputeranga MR on the Wellington South Coast to evaluate juvenile and adult population densities and examine stage-specific habitat requirements. Juvenile paua were found in higher densities at fished sites in areas that were sheltered from wave exposure and dominated by cobbles and boulder fields. Adult paua were found in greater densities and were larger in size within the reserve than outside, which was the opposite finding to the baseline survey illustrating reserve effectiveness. Although within the reserve there were large aggregations and individual adults which may support population reproductive success, juvenile and adult population densities were not correlated. Results from this study indicate that marine reserve implementation does have an impact on adult populations but that habitat is more important for juvenile success.

Although this thesis focused on paua within the scope of protection, MRs are placed in NZ to protect a suite of species. To thoroughly investigate habitats I conducted a rigorous inside-outside habitat analysis utilizing multibeam bathymetric data and video footage from drop camera surveys at Taputeranga MR. Habitat maps produced by NIWA were utilized to plan drop camera sampling locations and 278 drops were conducted across 8 sites associated with TMR. Analysis revealed that habitats within fished and reserve sites were comparable in physical and biogenic habitat quantities, although the reserve had greater topographic relief. However, when examining only a subsample of fished sites there were pronounced habitat differences between in and outside the reserve, where the western fished sites have significantly more rocky reef with greater algal cover than the reserve and eastern sites. These results illustrate the need for quantification of habitat when siting fished (control) areas and conducting inside versus outside reserve comparisons.

This research has determined that MRs do afford protection for paua in central NZ. The differentiation between habitat and reserve effects that I have identified has direct relevance to current and future MRs in NZ and highlights the need for studies to examine habitat effect in MR spatial planning at a global level. Furthermore, this research highlights the importance of considering stage-specific habitat requirements when designing the spatial arrangement of MRs by protecting juvenile habitat as well as adults to increase chances of recovery. These abalone-habitat associations, showing the importance of exposure and macroalgal cover for growth, can be used to assist in management decisions within NZ such as considerations for siting management areas and potential translocations and are directly applicable to abalone conservation, management concerns and recovery efforts across the world.