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Using length frequency distribution data (LFD) is cost-effective for estimating somatic growth in fish or invertebrates as length data are relatively easy to obtain. The recently developed R packages TropFishR and fishboot extend classic ELEFAN (Electronic LEngth Frequency ANalysis) programs and include more powerful optimization procedures and a bootstrap method for estimating uncertainties. Yet, the fundamental functions require users to provide search conditions (e.g. upper and lower limits for each parameter, length-class size, number of length-classes for the calculation of moving average), which can significantly affect the results. In this paper, we compare the ELEFAN approach with a Bayesian approach in analysing LFD, employing both standard and seasonal von Bertalanffy growth functions. We apply both approaches to a commercially valuable but poorly studied red endeavour prawn (Metapenaeus ensis) harvested in Australia's Northern Prawn Fishery. Sensitivity tests on ELEFAN confirm that any change in search settings would affect the results. Simulation studies on Bayesian growth models show that Linf and K can be accurately obtained even with modal progression of only one year-class and using non-informative priors. However, age information, including the theoretical age at length zero (t0), is difficult to estimate and requires LFD from multiple age classes and informative priors. The Bayesian models yield mean parameters of: Linf = 36.56 mm (carapace length), K = 2.74 yr–1, and t0 = -0.02 yr for the males, and Linf = 51.81 mm, K = 1.94 yr–1, and t0 = -0.02 yr for the females. Seasonal oscillation models fit the LFD better, but the improvement is small and the estimated season-related parameters have large variances. [ABSTRACT FROM AUTHOR]

Fisheries based on short lived species are notoriously difficult to manage using traditional effort or catch quota controls. Such fisheries are often characterised by high fecundity, requiring only a small spawning stock to potentially produce a large biomass the following year. This stock, or at least its availability, is driven by unpredictable environmental conditions to a large extent, with the available biomass depleted over the fishing season though both natural and fishing mortality. In most instances, management of these fisheries has focused on ensuring sufficient escapement at the end of the season to allow sufficient recruitment the following year. Catch-rate triggers are one such management tool to determine when fishing should cease. In this study, the use of such triggers in the banana prawn sub-fishery of the Australian Northern Prawn Fishery are assessed. The fishery has an explicit objective of achieving maximum economic yield, and the triggers are set on an economic basis during the season based on price and cost information provided by the industry. The in-season estimates provided by industry were highly correlated with information derived retrospectively through an economic survey, with the industry-provided estimates generally resulting in a more conservative (i.e., higher) break-even catch-rate level. It is the profit maximising behaviour of the fishers, however, that results in the greatest benefits, with most boats leaving the fishery before the trigger catch-rate is reached. The catch-rate trigger operates more as a "safety-net" for the fishery, and possibly a nudge encouraging vessels to maximise their profits given their individual cost structures. [ABSTRACT FROM AUTHOR]

Fisheries management must address multiple, often conflicting objectives in a highly uncertain context. In particular, while the bio-economic performance of trawl fisheries is subject to high levels of biological and economic uncertainty, the impact of trawling on broader biodiversity is also a major concern for their management. The purpose of this study is to propose an analytical framework to formally assess the trade-offs associated with balancing biological, economic and non-target species conservation objectives. We use the Australian Northern Prawn Fishery (NPF), which is one of the most valuable federally managed commercial fisheries in Australia, as a case study. We develop a stochastic co-viability assessment of the fishery under multiple management objectives. Results show that, due to the variability in the interactions between the fishery and the ecosystem, current management strategies are characterized by biological and economic risks. Results highlight the trade-offs between respecting biological, economic and non-target species conservation constraints at each point in time with a high probability and maximizing the net present value of the fishery. [ABSTRACT FROM AUTHOR]

Punt, A. E., Deng, R. A., Dichmont, C. M., Kompas, T., Venables, W. N., Zhou, S., Pascoe, S., Hutton, T., Kenyon, R., van der Velde, T., and Kienzle, M. 2010. Integrating size-structured assessment and bioeconomic management advice in Australia's northern prawn fishery. – ICES Journal of Marine Science, 67: 1785–1801.Three species in Australia's northern prawn fishery (Penaeus semisulcatus, P. esculentus, and Metapenaeus endeavouri) are assessed using a size-structured population model that operates on a weekly time-step. The parameters of this multispecies population model are estimated using data on catches, catch rates, length frequency data from surveys and the fishery, and tag release–recapture data. The model allows for the technical interaction among the three species. The results from the multispecies stock assessment are used to calculate the time-series of catches and levels of fishing effort that maximize net present value. The bioeconomic model takes into account costs which are proportional to catches and those which are proportional to fishing effort, as well as fixed costs. The sensitivity of the results is examined by changing the assumptions regarding the values for the economic parameters of the bioeconomic model as well as those on which the assessment are based. The results suggest that fishing effort needs to be reduced in the short term to achieve economic goals, although most stocks are estimated currently to be above the stock size corresponding to maximum sustainable yield. Short-term catches and effort levels are sensitive to model assumptions, and in particular, to trends in prices and costs. [ABSTRACT FROM PUBLISHER]

Venables, W. N., Ellis, N., Punt, A. E., Dichmont, C. M., and Deng, R. A. 2009. A simulation strategy for fleet dynamics in Australia’s northern prawn fishery: effort allocation at two scales. – ICES Journal of Marine Science, 66: 631–645. We describe simple and effective methods for simulating the movement of a fishing fleet, the spatio-temporal allocation of effort, in the context of harvest policy evaluation. Vessel movements are simulated in a manner that can balance established seasonal fishing patterns with the influence of perceived local variations in abundance. In the longer term, the model adapts smoothly to changes in fleet size and price fluctuations in critical resources, such as fuel. Given an initial allocation of effort at a fine temporal (but coarse spatial) scale, as needed for projecting stock dynamics, a second phase assigns the effort at a finer spatial (but coarser temporal) scale for environmental impact assessment. The first phase is based on a discrete-state, time-inhomogeneous Markov chain, with transition probabilities following a multiple logistic model. Although the ideas are generic, the method has been developed for the specific case of Australia’s northern prawn fishery, which is used to motivate and describe the method. The approach, in a harvest policy evaluation context, is novel in that it uses economic projections (not surrogates) to reflect realistically the recent pressures many fishing industries are under to change their spatial fishing pattern in response to global fuel price fluctuations. [ABSTRACT FROM PUBLISHER]

Formal harvest strategies (HS) confer increased certainty for stakeholders in decision-making processes because they include pre-agreed, transparent rules that can be simulation tested prior to implementation. An important trade-off for small, low-value fisheries relates to balancing the value of the fishery with the costs of data collection, analysis and completing a stock assessment. Harvest strategies also need to effectively balance risks to the fish stock with the objective of maximising yield or economic returns from a fishery. As part of this balancing act, short-lived highly-variable stocks present some unique challenges that need to be accounted for in a HS. We describe the process used to revise the HS for Australia's redleg banana prawn fishery (Penaeus indicus) to address these challenges. The fishery is managed using input controls and recently implemented a seasonal closure (based on management strategy evaluation testing) to improve the robustness of the HS to extreme environmental variability while providing ecological and economic benefits. An additional positive trade-off was that this conservative measure simplified the HS and added a risk buffer to counter uncertainties due to relying solely on catch-per-unit-effort (CPUE) data as an index of relative abundance. The stock assessment model and associated hockey-stick control rule were modified to account for the high natural variability of the stock by stipulating that closure of the fishery in any given year is only deemed necessary if the limit reference point is breached in two consecutive years prior. In years with low fishing effort (whether due to ecological or economic factors), it may not be necessary to conduct a stock assessment or there may not be sufficient data to inform a reliable assessment. We analysed fishery data to inform selection of a data-sufficient cut-off threshold that triggers whether or not a stock assessment is required in any particular year. We also considered potential risks to the fishery of not doing an assessment as well as the impact of this decision rule on the performance of future stock assessments. Finally, we summarise some general lessons around pragmatic design of a HS for a small, highly variable fishery needing to balance risk to the stock with optimising catches and reducing management costs. • Risk-catch-cost frontiers are important considerations in fisheries management. • Pragmatic simple harvest strategies are needed for small data-poor fisheries. • A data-sufficient cut-off number can be used to trigger a stock assessment. • Precautionary risk buffers can reduce management costs. • Short-lived highly variable fisheries benefit from tailored harvest strategies. [ABSTRACT FROM AUTHOR]

The Northern Prawn Fishery (NPF) is one of Australia's most valuable federally managed fisheries. It has also been a focus for modelling in Australia, with over 40 years of development of bioeconomic, stock assessment and management strategy evaluation models that have been used to varying degrees to inform management. The current management model has been in operation for over 10 years and is used to set the total allowable effort (TAE) for the tiger prawn (Penaeus semisulcatus and P. esculentus) component of the fishery, historically undertaken after a mid-year closure. The success of management has resulted in increased stocks of tiger prawns, and a fishery before the mid-year closure during which fishing for banana prawns was historically the main activity. While the current management model includes the consequences of fishing for tiger prawns during the first season in terms of which weeks are available for fishing for tiger prawns when estimating the TAE, the lack of explicit inclusion of banana prawns in the bioeconomic modelling has resulted in concerns that the TAE may be biased, as effort is allocated in the model to tiger prawns that, in reality, should be applied to banana prawns. The current model was consequently extended to include an explicit banana prawn component, and a range of scenarios undertaken to explore the impact of this on the management advice. It was found that the current model overestimates the optimal level of effort to be applied to tiger prawns except during periods of very low banana prawn abundance. Banana prawns, which vary substantially inter-annually due largely to environmental drivers, were also found to be the key driver of the level of overall fishery profit, suggesting that greater attention to this component of the fishery may be necessary to achieve the objective of maximizing net economic returns from the NPF. [ABSTRACT FROM AUTHOR]

Water resource development can lead to the significant alteration of natural flow regimes, which can have impacts on the many aquatic species that rely on both freshwater and estuarine environments to successfully complete their lifecycles. In tropical northern Australia, annual catches of commercially harvested white banana prawns (WBP) are highly variable in response to environmental conditions, namely rainfall and subsequent riverine flow. However, little is known about the spatial extent to which flow from individual rivers influences offshore WBP catch. In this study, we quantify how the relationship between WBP catch in the Gulf of Carpentaria is influenced by flow from the Mitchell River, Queensland Australia. We used a Bayesian framework to model both prawn presence and catch per unit effort, and found evidence that multiple components of the flow regime contribute to fishery catch. We also found evidence to suggest that the relationships between prawn presence and flow were spatially structured across the fishing ground. Our results suggest that attributing fishery catch to a single river remains challenging, though highlights the importance of maintaining natural flow regimes to support a highly valuable commercial fishery species in the face of potential water resource development. • Water resource development threatens natural flow regimes. • We modelled how riverine flow influences a highly valuable prawn fishery. • Multiple aspects of the flow regime were important for fishery catch. • Relationships between catch and flow were spatially structured in the fishery. • Our results inform management of ecosystems threatened by hydrologic development. [ABSTRACT FROM AUTHOR]

• Redleg banana prawns are caught in a remote part of the northern prawn fishery. • Fishing effort on the species has varied considerably in recent years. • We model the key drivers of fishing effort on the sub-fishery. • We find that economic conditions in the main part of the fishery is a key driver. Recent low levels of fishing effort in the redleg banana prawn (Penaeus indicus) component of the Australian Northern Prawn Fishery have highlighted challenges in assessing the stock status of the species and deriving appropriate management responses. Usually, low levels of fishing effort are associated with low stock abundance, but economic factors also affect fishing activity. In this study, we consider the key drivers of fishing effort in the sub-fishery, and conclude that fishing effort is driven by expectations of catch based on environmental drivers, realised revenue per unit of fishing effort, and the opportunity cost of fishing in other areas. In the case of the recent low effort levels, it is the opportunity cost that has been most influential, highlighting the importance of considering economic drivers when managing fisheries. [ABSTRACT FROM AUTHOR]