Developing population models for informing the sustainable management of the Murray spiny crayfish (Euastacus armatus) and the Glenelg spiny crayfish (E. bispinosus) in Victoria Potts, J.M.1 and C.R. Todd Arthur Rylah Institute for Environmental Research, 123 Brown St, Heidelberg 3084, Victoria. 1E-Mail: [email protected] Keywords: freshwater crayfish, population model, uncertainty, model structure. EXTENDED ABSTRACT We found that data on both species were lacking, including vital information about their life history Population models can be a useful tool for characteristics. In particular, it remains unclear identifying knowledge gaps to guide future whether moulting impedes the capacity of females research, ranking different management scenarios, to reproduce. Therefore, we propose two stage- and assessing the risk of extinction and based model structures to accommodate this conservation status of a target species. The process uncertainty. The ‘non-moult’ model defines two involves specifying a set of rules based on the life protected stages (ie. below the legal limit): history of the species that govern how the number juveniles, that are reproductively immature, and and distribution of individuals within the protected adults (that are reproductively mature). population change over time. There is one un-protected stage (ie. above the legal limit), that contains reproductively mature adults Data availability and quality can be a limiting available for harvesting. The final stage contains factor in model development, influencing the individuals who have died from harvesting (as estimation of parameters and the understanding of distinct from natural mortality). In addition, the important environmental processes. In such ‘moult’ model contains an additional two stages circumstances, uncertainty arises from a plethora for moulting females. In each time step, of sources: model structure, parameter estimation, individuals either progress to the next stage, or intra- and inter-species dependencies, and shape remain in their current state. Offspring of the uncertainty about a distribution. Choosing an reproductive stages enter the juvenile stage. Both appropriate model can therefore be problematic, model structures are sexually dependent, implying and should be governed by the life history a total of eight stages are required for the ‘non- characteristics of the target species, knowledge of moult’ model and ten stages for the ‘moult’ model. environmental processes and the amount of data available. By fixing some transition rates between stages to equal zero, a variety of model constructs can be Here, we describe a conceptual population model obtained from the two species models. We propose for two species of freshwater crayfish, the Murray five constructs to accommodate species-specific spiny crayfish (Euastacus armatus) and the management actions. For example, all female Glenelg spiny crayfish (E. bispinosus). Both E. bispinosus are protected. Therefore, any species are listed nationally as ‘vulnerable’. They transitions out of the protected female adult stage are not listed in Victoria despite substantial (other than natural mortality) equal zero. reductions in range and population abundance that have resulted in local extinctions. Identified threats Considerable uncertainty remains as to which to both species include overfishing by recreational particular model structure would be most useful anglers and habitat modification. Restrictions are for informing management about the efficacy of currently in place that govern the time of year, the current regulations. However these conceptual number, sex and size of individuals that can be models offer valuable insight, by highlighting removed. However these regulations have not current knowledge gaps. The next stage of the halted the observed decline in catch size and modelling process will be to collect empirical data abundance, as reported by local fisherman. on the species such that these models can be parameterised. 367 whether improved knowledge or greater accuracy 1. INTRODUCTION would change the decision-making process and consequently the management of the resource Australia contains approximately 20% of the (Starfield 1997, Bearlin et al 2002). world’s freshwater crayfish species and subspecies (Taylor 2002). Since European settlement, the The modelling process characterises the attributes range and abundance of these species has declined that cause a population to change: a set of logical (Barker 1990). Of particular conservation rules specifying how the number and distribution importance are the Euastacus armatus (the Murray of individuals within a population change over spiny crayfish) and E. bispinosus (the Glenelg time (Easterling et al 2000). Because modelling spiny crayfish) species which are listed nationally explicitly deals with uncertainty, subjective as ‘vulnerable’ and are protected in South opinion is removed from the management decision Australia and the ACT (Clarke and Spier-Ashcroft making process (Todd et al 2004). 2002). However neither E. armatus and Requisite data include state variables that describe E. bispinosus are listed in Victoria, despite having the population (i.e. life history characteristics such suffered a documented range reduction as well as as rates of birth, growth, maturation, fertility and localised population extinctions (Horwitz 1990). mortality; immigrations and emigrations) and The main threats to both crayfish species is process variables (i.e. the environmental processes overfishing by recreational anglers (Horwitz 1990, that invoke a response in the population). Process Lintermans and Rutzou 1991) and habitat variables include stochastic events that are modification (Clarke and Spier-Ashcroft 2002). unpredictable changes in the state of the system. While the occurrence of habitat modification has They include demographic stochasticity, reflecting greatly diminished in recent years, the impacts of the uncertainty associated with each individual; recreational angling have not. Recently, the environmental stochasticity, which are fluctuations importance of leaving larger individuals in fishery in the environment caused by temporal variation of management has become more frequently habitat parameters; genetic stochasticity, caused by discussed for its implications in the sustainable inbreeding and genetic drift; and catastrophic harvesting of exploited species (Birkeland and stochasticity, which are extreme environmental Dayton 2005). Despite this, the removal of larger fluctuations (Possingham et al 2001). In order to individuals has been recognised as a threat to the parameterise a population model, the relationship sustainable management of E. armatus (Horwitz between the process variables and the state 1990, Lintermans and Rutzou 1991). variables must be estimated. Although the fishery is subject to regulation, there In conjunction with these four stochastic processes are no measures in place to collect information on that are known to affect populations, a variety of the number of crayfish being removed, or to different types of uncertainty also exist: structural provide information on the efficacy of the current uncertainty; parameter uncertainty; dependency regulations upon the long-term persistence of uncertainty; and shape uncertainty about a either species. The aim of this paper is to examine distribution (Todd 2001). We can characterise the life history of freshwater crayfish in order to these uncertainties in two ways: epistemic develop conceptual population models for both uncertainty resulting from incomplete information E. armatus and E. bispinosus. Once these models about the system in question (ignorance); or the have been parameterised through empirical data underlying or inherent stochasticity in the system collection, better informed management decisions (variability) (Ferson and Ginzburg 1996). can be made by ensuring the process is transparent, explicit and repeatable. 3. BIOLOGY OF THE FRESHWATER SPINY CRAYFISH 2. POPULATION MODELLING AS AN E. armatus, E. bispinosus and Astacopsis gouldi EXPLORATORY TOOL (occurring in Tasmania) are collectively known as Population models are useful within the context of large lowland spiny crayfish (Horwitz 1990). The resource management and decision making to preferred habitat of these species is cool and clean assess conservation status, guide future research flowing waters with intact riparian vegetation and rank alternate management strategies, (Barker 1990). E. armatus occurs the Murray particularly in circumstances of incomplete data or River throughout most of its length and major lack of full ecological knowledge (Todd et al tributaries in Victoria and New South Wales, and 2002, Todd et al 2004). Modelling can also E. bispinosus occurs in the Glenelg River and its identify the sensitivities associated with tributaries in Western Victoria and a small portion uncertainty about particular mechanisms, and of South Australia (Morgan 1986). 368 The freshwater crayfish E. armatus and adult form (despite being sexually immature) by E. bispinosus share a similar life history: adults approximately 170 days and are released in spring produce eggs, eggs hatch to become juveniles that to summer (O'Connor 1984, Reynolds 2002). grow and develop into adults. Both species are It is not clear from the literature whether moulting slow growing and long lived, achieving relatively impedes the capacity of females to reproduce. large sizes: an orbital carapace length (OCL) of