Qualitative mathematical models to support ecosystem-based management of Australia's Northern Prawn Fishery

A major decline in the catch of the banana prawn [shrimp], Penaeus (Fenneropenaeus) merguiensis, occurred over a six-year period in the Weipa region of the northeastern Gulf of Carpentaria, Australia. Three main hypotheses have been developed to explain this decline: (1) prawn recruitment collapsed due to overfishing; (2) recruitment collapsed due to a change in the prawn's environment; and (3) adult banana prawns were still present, but fishers could no longer effectively find or catch them. Qualitative mathematical models were used to link population biology, environmental factors, and fishery dynamics to evaluate the alternative hypotheses. This modeling approach provides the means to rapidly integrate knowledge across disciplines and consider alternative hypotheses about how the structure and function of an ecosystem affects its dynamics. Alternative models were constructed to address the different hypotheses and also to encompass a diversity of opinion about the underlying dynamics of the system. Key findings from these analyses are that: instability in the system can arise when discarded fishery bycatch supports relatively high predation pressure; system stability can be enhanced by management of fishing effort or stock catchability; catch per unit effort is not necessarily a reliable indicator of stock abundance; a change in early-season rainfall should affect all stages in the banana prawn's life cycle; and a reduced catch in the Weipa region can create and reinforce a shift in fishing effort away from Weipa. Results from the models informed an approach to test the hypotheses (i.e., an experimental fishing program), and promoted understanding of the system among researchers, management agencies, and industry. The analytical tools developed in this work to address stages of a prawn life cycle and fishery dynamics are generally applicable to any exploited natural resource

Phytoplankton community structure and productivity in relation to the hydrological regime of the Gulf of Carpentaria, Australia, in summer

To describe the influence of hydrology on the phytoplankton communites of the Gulf of Carpentaria, six phytoplankton parameters were measured on five transects: productivity, biomass, community structure, phospholipid-derived fatty acids, ratios of stable carbon isotopes and the relationship between photosynthesis and irradiance. The mean (±s.e.) estimate of depth-integrated phytoplankton productivity was 914 ± 185 mgC m-2 day-1 (n = 9). In the shallow ( 10 fim, with little in between. The highest rates of growth occurred within a very narrow light regime: self-shading limited growth in deeper water (at light intensities less than 125 mol s-1 m-2), and growth was photoinhibited in shallow water (at light intensities greater than 150 mol s-1 m-2). The resulting biomass-specific primary productivity (mgC mgChla-1 day-1) maximum did not coincide with the depth of either the chlorophyll a maximum or the highest nutrient concentration. The natural carbon isotope ratio for the particulate matter showed that little land-derived carbon was exported beyond a narrow coastal fringe (about 10 to 20 km). The profiles of phospholipid fatty acids also showed that particulate organic matter was dominated by algal-derived compounds, which confirms that the bulk of the organic matter in the offshore Gulf of Carpentaria is of marine origin. Therefore, during the summer in this study, the stratified waters of the central gulf were both biologically and hydrologically independent of the coastal water and not influenced by terrestrial runoff. The phytoplankton production in the central gulf is maintained by nutrient supplies from internal sources, such as those that are remineralized in and resuspended from the sediment

Seasonal, interannual and spatial variability in the reproductive dynamics of Penaeus merguiensis

Penaeid prawns (shrimp) are short-lived and fecund, with a complicated life cycle that includes offshore spawning followed by a coastal or estuarine postlarval and juvenile phase. Factors affecting survival during the early life-history stages, and during movement between these stages, will affect variability in recruitment to the nursery ground, the offshore subadult and adult population, and, ultimately, catch. The inability to predict recruitment, and ultimately commercial offshore catch, has been complicated by an incomplete understanding of these factors. The reproductive dynamics of Penaeus (Fenneropenaeus) merguiensis were investigated by simultaneous adult and larval sampling on 66 lunar-monthly surveys from March 1986 to March 1992 in Albatross Bay, northeastern Gulf of Carpentaria, Australia. Egg production was seasonal, with the highest production from 6-mo-old newly recruited spawners, and another peak from 12mo-old spawners. Larval abundance (no. m-2) followed the same seasonal pattern as the abundance of eggs. However, interannual variation in egg and larval abundance was large, and there was a weak correlation between monthly egg and larval abundance. Larval abundance appeared to be further influenced by fluctuations in chlorophyll a concentration, a measure of food availability. There was evidence of a match/mismatch relationship between larval abundance and episodic chlorophyll increases. While there was no direct spawner (egg production)-fishery recruit relationship in P. merguiensis over the 6-yr study, there was a strong relationship between total larval abundance in spring and the size of the commercial catch 3 to 6 mo later. Therefore, factors affecting larval survival, including food availability, have significant implications for fishery production

Advances in Marine Biology V49: Restocking and stock enhancement of marine invertebrate fisheries

Description Many of the world’s fisheries are in trouble - they no longer yield the catches, and potential profits, they once did. The habitats that support fisheries have been damaged by pollution and other irresponsible use of coastal land. Destructive fishing methods like trawling and blast fishing have also changed fish habitats resulting in support of fewer fish. The authors draw on more than 1000 scientific papers covering 11 groups/species of marine invertebrates. From this large literature, they distill 20 lessons for assessing and guiding the use of restocking and stock enhancement in the management of invertebrate fisheries