Developing population models for informing the sustainable management of the Murray spiny ( armatus) and the Glenelg spiny crayfish (E. bispinosus) in

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 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 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 (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 Female A. gouldi moult and reproduce in alternate 180mm and 2.7kg for E. armatus (O'Connor years (Reynolds 2002). Many species of the family 1984); and an OCL of 130mm and 1.14kg for Astacidae (including the Euastacus genus) have E. bispinosus (Horwitz 1990). Growth rates of biennial breeding, however E. bispinosus has been crayfish depend on two factors, the frequency of observed to breed in successive years without moulting and growth increment at each moult. It moulting (Honan and Mitchell 1995c). O’Connor has been documented that the estimated age of an (1984) states that E. armatus reproduces annually, E. armatus with an OCL of 100mm was between 7 where tagging data confirmed breeding in and 14 years old, with a likely age of 9-10 and subsequent years, but they did not indicate whether larger individuals living 20-25 years (O'Connor a moult had occurred in any of the observed 1984). E. bispinosus have been estimated to take females. While it appears that both E. armatus and 8-12 years to reach 100mm OCL (Horwitz 1990). E. bispinosus may reproduce annually, it remains Honan and Mitchell (1995b) aged a 130mm OCL unclear if moulting disrupts the reproductive crayfish at 25 years. output of females. In Euastacus species, sexual maturity is identified by changes in gonospore condition and the presences of eggs (Turvey 1980, Morgan 1986). 4. MODEL STRUCTURE The length at which sexual maturity is reached Choosing an appropriate model structure is varies for both E. armatus and E. bispinosus. determined by the life history of the target species Morgan (1986) states that for E. armatus female and the associated management objectives. The maturity occurs from 40 to 100mm OCL, and that current management objective for freshwater spiny very large specimens may be immature. Honan and crayfish species is to maintain the “sustainability Mitchell (1995a) record sexual maturity to occur of the resource as well as a satisfactory between 54.5 to 81mm OCL for E. bispinosus. recreational experience for users of the resource” Both species exhibit significant site variation such (Barker 1990). We cannot include a ‘satisfactory that individuals from smaller streams reach sexual recreational experience’ in our model, although maturity at smaller sizes. this may be achievable depending on a suitable definition. As a result, we will focus on a model The clutch size of freshwater spiny crayfish can be structure that will allow questions regarding the estimated from counts of eggs or young, and sustainable exploitation of E. armatus and generally increases with increasing female body E. bispinosus to be addressed. size. Clutch sizes for E. bispinosus ranged from 63 eggs on a 62mm OCL female to 812 eggs on a Although some information exists regarding the 115mm OCL female (Honan 1990). O’Connor freshwater spiny crayfish (e.g. egg numbers-size (1984) estimated a female E. armatus individual relationship) many life history aspects remain with a 90mm OCL to produce 480 eggs and a poorly understood (e.g. survival rates). At present, 125mm OCL individual to produce 1815 eggs. the life-history characteristics for both species are poorly known. In order to include uncertainty Clutch sizes are highly variable for several about the interaction between reproduction and reasons: if food is restricted, ovarian development moulting two model structures were developed. can be limited; depth, light and temperature affect They account for Euastacus species reproducing reproductive success; and damage and disease every year (‘non-moult’, Figure 1) or only cause egg mortality. By the end of the incubation reproducing in non-moult years (‘moult’, Figure period, up to 50% of females may have lost some 2). Parameters in the model include: or all of their brood (Honan 1990) • All Euastacus species reproduce sexually, mostly Si,j – Survival (transition from stage i at spawning in late summer to autumn. This is time t, to stage j at time t+1) followed by winter brooding of the eggs, where • Rj,i – Reproduction (number of individuals females keep the eggs under their tail until they that contribute to stage i at time t+1, from hatch. Eggs hatch in October to November but stage j at time t) remain attached to the female parent by a filamentous oosetae. During this period juveniles • H – Harvesting (transition of individuals experience three moulting stages. Individuals are into the harvested population) fully independent and eventually resemble the • Prefixes: F= female and M=male

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These two model structures are sex dependent. The MR FR3,1 3,1 transition from adult to one year old juveniles, MR2,1 implicitly accommodates fecundity, egg survival FR2,1 (hatching), and survival of juveniles less than one year old.

FS1,1 FS2,2 FH FJ P-FA NP-FA H-FA By modifying transition rates between stages in these two models, species-specific constructs can FS3,3 FS1,2 FS2,3 be developed. This is necessary because current fishing regulations differ for the two species: • E. bispinosus - A minimum carapace MS MS MH MJ 1,2 P-MA 2,3 NP-MA H-MA length (OCL) of 10cm and only one (male) crayfish per day. There is no closed season. MS1,1 MS2,2 MS3,3 • E. armatus - A minimum carapace length Figure 1. Non-moulting model for the freshwater of 9cm, and up to 5 crayfish per day where crayfish: juveniles (J) can remain in the same stage only one may exceed 12cm. There is a closed season for Victorian waters north of (S1,1), grow into reproductive adults that are too the Great Divide from September to April, small to be harvested (protected adults, P-A) (S1,2), or die; protected adults can reproduce to create all-inclusive. juveniles (F2,1), remain in the same stage (S2,2), • Both species – No removal of female grow into harvested, non-protected adults (NP-A) crayfish that are in ‘berry’. (S2,3) or die; harvested adults can reproduce to create juveniles (F3,1), remain in the same stage In total, five model constructs are proposed that (S3,3), or die (either from natural causes, or accommodate these various management actions harvesting at rate H). Prefixes: F=female and (Table 1). M=male. 4.1. Construct One

This is the simplest model construct that explicitly accounts for harvesting. It is applicable for situations where males are believed to be a non- MR 3,1

FR3,1 MR2,1 limiting factor (i.e. response of female population FR2,1 can be assumed to reflect the response of the entire population). For the Murray crayfish, this

FS 1,2 FH FJ P-FA NP-FA H-FA assumption may be valid, as female individuals are available to harvesting. The model is not

FS1,1 FS 2,2 FS3,3 applicable to the Glenelg spiny crayfish, as all FH females are protected under current fishing P-A(m) NP-A(m) FS2,3 regulations.

MS 1,2 MS2,3 MH MJ P-MA NP-MA H-MA 4.2. Construct Two and Three These models build on the previous construct by MS 1, 1 MS 2,2 MS 3,3 explicitly accounting for harvesting in the male Figure 2. Moulting model for the freshwater and female populations. This is applicable to crayfish. As per Figure 1, with two additional situations when harvesting rates of both sexes may transitions from protected female adults (P-FA) to differ. In populations of freshwater crayfish, this protected female adults in a moulting period (P- may occur as regulations prohibit the removal of FA(m)) and from non-protected female adults (NP- female crayfish that are in ‘berry’. FA) to non-protected female adults in a moulting Construct two is appropriate for E. armatus as it period (NP-FA(m)). Prefixes: F=female and allows for harvesting of the female population. M=male. Given current fishing regulations protect all E. bispinosus females, construct three is required that does not account for the removal of females from the population via harvesting.

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Table 1. Construct definitions. If boxed is ticked, that parameter is included in the construct. For example, FS1,1 is needed in construct 1.

Construct FS1,1 FS1,2 FS2,2 FS2,3 FS3,3 MS1,1 MS1,2 MS2,2 MS2,3 MS3,3 FR2,1 FR3,1 MR2,1 MR3,1 FH MH

1 9 9 9 9 9 − − − − − 9 9 − − 9 − 2 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 3 9 9 9 9 9 9 9 9 9 9 9 9 9 9 − 9 4 9 9 9 9 9 − − − − − 9 9 − − − − 5 9 9 9 9 9 9 9 9 9 9 9 9 9 9 − −

This study has also identified the need for further 4.3. Construct Four and Five research to understand the influence of moulting on the reproductive process. We do not believe These models do not explicitly account for that it would be difficult to establish whether harvesting, instead assume that estimates of moulting influences the reproductive process. survival incorporate the affect of harvesting. They will be easier to parameterise than the other There are at least two numerical approaches to models because less transition estimates are modelling the exploited populations of these required, however they cannot explicitly species, a conservation/population viability investigate management questions related to approach (Todd et al 2001, Todd et al 2002) and a harvesting pressure. Construct four is a female fisheries/exploitation approach (Punt and only model (e.g. E. armatus), and construct five is Kennedy 1997). The structures identified in this a sex-dependent model (e.g. E. bispinosus). study are most suited to being modelled in the conservation/population viability approach as

fishing is explicitly modelled. This approach also incorporates demographic stochasticity. These 5. DISCUSSION structures can be easily modified to suit the fisheries/exploitation approach by implicitly Considerable uncertainty remains as to which accounting for fishing through adjusting survival particular structure would be most useful for in the unprotected adult stage by the rate of informing management about the efficacy of the removal. Neither approach incorporates genetic current regulations and whether management is stochasticity, which has not been identified as a meeting stated objectives. In order to assess any management priority to date. of the structures identified in this study, parameters need to be estimated from data. Very little data has been collected on the either 6. CONCLUSION E. armatus or E. bispinosus, and no data has been The population models developed are a collected in the past decade. Inadequate amounts reasonable summary of the life history of the of data have been problematic in attempts at species and could be used as an exploratory building models for solving real-world problems management tool. For the model to be of most (Akçakaya and Burgman 1995, Burgman and benefit to management, this would require the Possingham 2000). parameters to be estimated through further Advances in tagging technology means that research. The construction of the model highlights freshwater spiny crayfish could be individually where the knowledge gaps are that would most marked and the tag retained through the moulting usefully inform management about the impacts process (M. Lintermans, pers. com., Montgomery from recreational fishing. These were identified as and Brett 1996). Consequently, data that directly estimation of transition rates, reproductive rates, lend itself to being used in population modelling harvesting rates, size at sexual maturity, and could be collected in the future through a growth/age relationships. By identifying comprehensive mark-recapture program. knowledge gaps, this may contribute to an efficient allocation of resources, as funding can

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