Lobster movement ecology, energetics and conservation

 

Tracking lobsters using acoustic telemetry

Both commercial and recreational fishing have been banned in the Cape Rodney-Okakari Point Marine Reserve since 1977, in part to protect vulnerable and commercially valuable species, such as lobsters and snapper. The lobster population inside the reserve initially increased, but in the mid-1990’s began to decline to pre-reserve levels. Research on lobster movements in the reserve has previously documented that animals can leave the reserve boundaries, particularly in the offshore direction, during certain seasons and life history stages. These offshore movements may be facilitating the removal of lobsters by the local fishery, and diminishing numbers of adult lobsters in the reserve. In this study, we aim to characterize lobster movements within the reserve in much higher detail than was previously possible, and
quantify the number, sizes and sexes of lobsters moving across the offshore reserve boundary during different seasons. To do this, we will capture lobsters using commercial pots (traps), attach acoustic tags to their backs, and release them back into the wild. The tags will enable us to identify individuals and record their movements using an underwater array of receivers that “listen” for each tag’s signal. This research will contribute to our understanding of lobster ecology, but will also provide valuable information that will guide the conservation and management of the species.

 

Comparing lobster population sizes, growth and mortality rates, and nutritional condition between marine reserves

Declining lobster populations are not unique to the Cape Rodney-Okakari Point (CROP) Marine Reserve. In fact, lobster densities throughout New Zealand's North Island are decreasing, negatively affecting both the health of coastal reef ecosystems and the success of commercial lobster fishermen, who historically have composed the country's most valuable inshore fishery. In this project, my colleagues and I have partnered with the Department of Conservation to survey lobster populations in the CROP, Tawharanui, and Te Tapuwae o Rongokako (Gisborne) Marine Reserves. In order to better understand the relative roles of fishing pressure and population dynamics in lobster population decline, we are quantifying the density, growth rate, mortality rate and nutritional health of lobsters of different sexes and sizes within the marine reserves, and determining how these characteristics vary with seasonal movements of lobsters towards and away from the reserves' boundaries. This research will help us understand how lobster demographics differ between reserves, and will also guide local conservation and management of the species.

 

Modeling lobster population dynamics under varying fishing pressure and recuitment conditions

Marine reserves provide protection to harvested species within their boundaries, and can reverse the ecosystem-level effects of fishing pressure. This has been well documented in the Cape Rodney-Okakari Point (CROP) Marine Reserve in northern New Zealand. However, marine reserves may be less effective in protecting mobile species that experience low or highly sporadic recruitment, especially when those reserves are small. As part of this project, my colleagues and I have synthesized recent monitoring data from the CROP Marine Reserve and two other nearby reserves, showing large-scale declines in populations of the spiny lobster Jasus edwardsii. Current lobster biomass within the reserves is less than 20% of historic levels, while biomass outside the reserves is only 1% of historic levels within the reserves. Adult J. edwardsii undertake seasonal migrations beyond the reserve boundaries, and so we hypothesized that the observed declines are likely a result of sustained fishing pressure targeting the reserve boundaries in concert with an extended period of low recruitment. To explore the effects of low recruitment on lobster density within the reserves, we are employing an agent-based demographic model. We plan to hindcast the levels of recruitment at each reserve over the past 10-20 years, and then estimate the recruitment levels that would have been required to sustain lobster densities given current fishing pressure. This research has already shown that for mobile species with low or highly variable recruitment, marine reserves must be large enough to encompass the totality of seasonal movements in order to be effective. It may also serve as a guide for local management, suggesting how catch rates might be adjusted based on recruitment rates.

 

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Benn Hanns with a lobster

Doctoral student Benn Hanns posing with a lobster before releasing it back to the wild.