Catherine M. Dichmont

On implementing maximum economic yield in commercial fisheries

Economists have long argued that a fishery that maximizes its economic potential usually will also satisfy its conservation objectives. Recently, maximum economic yield (MEY) has been identified as a primary management objective for Australian fisheries and is under consideration elsewhere. However, first attempts at estimating MEY as an actual management target for a real fishery (rather than a conceptual or theoretical exercise) have highlighted some substantial complexities generally unconsidered by fisheries economists. Here, we highlight some of the main issues encountered in our experience and their implications for estimating and transitioning to MEY. Using a bioeconomic model of an Australian fishery for which MEY is the management target, we note that unconstrained optimization may result in effort trajectories that would not be acceptable to industry or managers. Different assumptions regarding appropriate constraints result in different outcomes, each of which may be considered a valid MEY. Similarly, alternative treatments of prices and costs may result in differing estimates of MEY and their associated effort trajectories. To develop an implementable management strategy in an adaptive management framework, a set of assumptions must be agreed among scientists, economists, and industry and managers, indicating that operationalizing MEY is not simply a matter of estimating the numbers but requires strong industry commitment and involvement.

Modelling climate-change effects on Australian and Pacific aquatic ecosystems: a review of analytical tools and management implications

Climate change presents significant challenges to modelling and managing aquatic resources. Equilibrium assumptions common in many modelling approaches need to be replaced by formulations that allow for changing baselines and integration of ongoing changes and adaptations by species, ecosystems and humans. As ecosystems change, so will the ways humans use, monitor and manage them. Consequently, adaptive management loops and supporting tools deserve more prominence in the management toolbox. Models are critical tools for providing an early understanding of the challenges to be faced by integrating observations and examining possible solutions. We review modelling tools currently available to incorporate the effect of climate change on marine and freshwater ecosystems, and the implications for management of natural resources. System non-linearity can confound interpretations and hence adaptive management responses are needed that are robust to unexpected outcomes. An improvement in the ability to model the effects of climate change from a social and economic perspective is necessary. The outputs from ‘end-to-end’ and socio-ecological models can potentially inform planning, in both Australia and the Pacific region, about how best to build resilience to climate change. In this context, the importance of well directed data-collection programs is also emphasised. Lessons from this region, which is advanced with regard to modelling approaches, can guide increased use of models to test options for managing aquatic resources worldwide.

Maximizing Profits and Conserving Stocks in the Australian Northern Prawn Fishery

The Australian Northern Prawn Fishery (NPF) is one of the few that has adopted a dynamic version of a ‘maximum economic yield’ (MEY) target, and, on this basis, the fishery is undergoing a process of substantial stock rebuilding. This study details the bioeconomic model used to provide scientific management advice for the NPF, in terms of the amount of allowable total gear length in the fishery, for both the MEY target and the path to MEY. It combines the stock assessment process for two species of tiger prawns with a specification for discounted economic profits, where the harvest function in the profit equation is stock-dependent. Results for the NPF show a substantial ‘stock effect’, indicating the importance of conserving fish stocks for profitability. MEY thus occurs at a stock size that is larger than that at maximum sustainable yield, leading to a ‘win-win’ situation for both the industry (added profitability) and the environment (larger fish stocks and lower impact on the ecosystem). Sensitivity results emphasize this effect by showing that the MEY target is much more sensitive to changes in the price of prawns and the cost of fuel, and far less so to the rate of discount.

EDITOR'S CHOICE: Evaluating marine spatial closures with conflicting fisheries and conservation objectives

Summary Spatial management is used extensively in natural resource management to address sustainability and biodiversity issues, for example through declaration of terrestrial National Parks and marine protected areas (MPAs). Spatial management is used also to optimize yields or protect key parts of the life cycle of species that are utilized (hunted, farmed or fished), for example through rotational harvesting. To evaluate the effectiveness of marine spatial closures with conflicting fisheries and conservation objectives, a series of marine fisheries closures are here analysed using an integrative modelling tool known as management strategy evaluation (MSE). This modelling framework combines a food web model of a tropical ecosystem fished by a prawn (shrimp) fishery that emulates the resource being managed, together with the present management system and risk-based tools of fishing the prawn species at maximum economic yield. A series of spatial closures are designed and tested with the aim of investigating trade-offs among biodiversity (MPA), benthic impacts, ecosystem function, key species at risk to fishing, economic and sustainability objectives. Synthesis and applications. This paper illustrates that existing tools often available in actively managed fisheries can be linked together into an effective management strategy evaluation framework. Spatial closures tended to succeed with respect to their specific design objective, but this benefit did not necessarily flow to other broad-scale objectives. This demonstrates that there is no single management tool which satisfies all objectives, and that a suite of management tools is needed.

Shared Stocks of Snappers (Lutjanidae) in Australia and Indonesia: Integrating Biology, Population Dynamics and Socio-Economics to Examine Management Scenarios

Joint Australia–Indonesia scientific workshops on the fisheries of the Arafura Sea, held in 1992 and 1994, concluded that the two countries might share stocks of the red snappers Lutjanus malabaricus and L. erythropterus and the gold-band snapper Pristipomoides multidens. At that time, no information concerning stock structure, distribution and movements of these species was available. Moreover, data on the population biology and on commercial catches were inadequate. Such data are crucial for stock assessment and for managing the stocks. Clearly, if the stocks being fished were shared, joint management would be appropriate. In order to answer the questions related to managing shared stocks, a collaborative research project was initiated by Australia (CSIRO as the lead agency) and Indonesia in 1999. The objectives were firstly, to describe the population dynamics, stock structure and biology of snappers relevant to the management of stocks shared between Australian and Indonesian fisheries; secondly, to characterize the social and financial structures of the Indonesian fishery so they could be taken into account in the development of management strategies; and thirdly, to explore ways of developing complementary management for the long term sustainability of the snapper fisheries. This project finished in 2003 and in this paper we bring together the results of the biological, genetic, population dynamics and socioeconomic research in relation to managing shared stocks in the context of managed versus unmanaged fisheries, small scale and industrial fisheries, and in both developed and developing country regulatory environments. Severe data limitations necessitated an innovative approach making use of comparative analyses, often data-poor values, and the drawing together of fishery dependent and independent data to evaluate the status of the stocks.

Optimal Vessel Size and Output in the Australian Northern Prawn Fishery: A Restricted Profit Function Approach

Individual transferable quotas (ITQs) are to be introduced into Australias Northern Prawn fishery in the near future. Total allowable catches (TACs) are to be set with the objective of maximising economic efficiency in the fishery. Under ITQs, vessel owners have the ability to adjust their fishing activities to maximise profits and changes in fleet structure resulting from management changes need to be considered when determining TACs. A restricted profit function for the fishery was estimated to determine the optimal vessel characteristics and output levels as a guide to how the fleet may adjust under an ITQ system. Vessels were found to be currently close to their optimal size given average historic prices and current stock conditions. However, higher tiger prawn stocks are expected to result in the average size of vessels increasing, with rising fuel prices also likely to result in capital being substituted for fishing days. Optimal average vessel-level catches of the main species are lower than current average vessel catches for a wide range of input and output prices. These changes in vessel characteristics and behaviour need to be incorporated in the derivation of the optimal TACs if economic efficiency objectives are to be achieved.

Performance of methods for estimating size–transition matrices using tag–recapture data

Management advice for hard-to-age species such as prawns, crabs and rock lobsters are usually based on size-structured population dynamics models. These models require a size–transition matrix that specifies the probabilities of growing from one size-class to the others. Many methods exist to estimate size–transition matrices using tag–recapture data. However, they have not been compared in a systematic way. Eight of these methods are compared using Monte Carlo simulations parameterised using the data for the tiger prawn (Penaeus semisulcatus). Four of the methods are then applied to tag–recapture data for three prawn species in Australia’s Northern Prawn Fishery to highlight the considerable sensitivity of model outputs to the method for estimating the size–transition matrix. The simulations show that not all methods perform equally well and that some methods are extremely poor. The ‘best’ methods, as identified in the simulations, are those that allow for individual variability in the parameters of the growth curve as well as the age-at-release. A method that assumes that l∞ rather than k varies among individuals tends to be more robust to violations of model assumptions.

Modified hierarchical Bayesian biomass dynamics models for assessment of short-lived invertebrates: a comparison for tropical tiger prawns

Conventional biomass dynamics models express next year’s biomass as this year’s biomass plus surplus production less catch. These models are typically applied to species with several age-classes but it is unclear how well they perform for short-lived species with low survival and high recruitment variation. Two alternative versions of the standard biomass dynamics model (Standard) were constructed for short-lived species by ignoring the ‘old biomass’ term (Annual), and assuming that the biomass at the start of the next year depends on density-dependent processes that are a function of that biomass (Stock-recruit). These models were fitted to catch and effort data for the grooved tiger prawn Penaeus semisulcatus using a hierarchical Bayesian technique. The results from the biomass dynamics models were compared with those from more complicated weekly delay-difference models. The analyses show that: the Standard model is flexible for short-lived species; the Stock-recruit model provides the most parsimonious fit; simple biomass dynamics models can provide virtually identical results to data-demanding models; and spatial variability in key population dynamics parameters exists for P. semisulacatus. The method outlined in this paper provides a means to conduct quantitative population assessments for data-limited short-lived species.

Estimating prawn abundance and catchability from catch-effort data: comparison of fixed and random effects models using maximum likelihood and hierarchical Bayesian methods

Abundance and catchability are crucial quantities in fisheries management, yet they are very difficult to estimate, particularly for short-lived invertebrates. Using two distinct approaches - a standard non-hierarchical model (NH) and a hierarchical Bayesian model (HB) - abundance and catchability coefficients from a fishery depletion process for banana prawns (Penaeus merguiensis) in northern Australia were estimated. Non-hierarchical models treated each stock and year separately and individually, whereas the hierarchical models assumed some form of common underlying population from which the parameters for the individual cases generated by the combination of stock and year were drawn. Two HBs were considered. In HB1 it was assumed that annual abundance and catchability parameters came from separate populations, or distributions, for each stock. In HB2 it was assumed that these stock region distributions were not separate, but had their parameters drawn from a common distribution. Thus in HB2 all stocks shared information at the regional level. The results for both NH and HB methods were similar in most cases, indicating a fair degree of stability irrespective of the particular form of model chosen. However, the NH method suffered because the data were analysed in generally small sections and in many cases these sections were too small to allow precise estimation of both parameters and confidence intervals. The deviations of point estimates between the HB1, HB2 and NH models were more marked in catchability coefficient estimates than in abundance estimates, and large relative deviations typically occurred in stock regions and years with low fishing efforts, low catch or poor depletion trends over time. We conclude that the combined analysis using HB was superior because it could handle limited data, yielded credible interval estimates for all parameters and was computationally more efficient.

The cost of co-viability in the Australian northern prawn fishery

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.