Precautionary Approach in Development of a Directed Horse Clam Fishery In

Home , Tresus, Tresus capax, Tresus nuttallii

Precautionary approach in development of a directed horse clam fishery in

British Columbia, Canada

Z. Zhang Pac@c Biological Station, Department of Fisheries and Oceans,

Canada

Abstract

A directed fishery on horse clams is being developed in British Columbia,

Canada. The paper describes how the development follows the guidelines of a framework, which explicitly endorses the precautionary approach. At the first phase of the development, biological and fisheries information on horse clams, such as longevity, reproductive characteristics, growth, mortality, and management schemes, was synthesised based on scientific literature, technical reports, and surveys. Growth parameters and natural mortality rates were estimated in the literature with certain degrees of uncertainties. At the second phase of the development, alternative management strategies were evaluated and a precautionary approach for setting up biological reference points was provided through a simulation study. A stochastic spawning stock biomass per recruit model was found to be able to fully utilise the available information.

Uncertainties about the estimates of population parameters were incorporated in the simulation to produce a probability distribution of possible consequences in terms of reproduction potentials. After setting up a limit reference point, the fisheries managers are able to find from the probability distribution such an exploitation rate that the risk of reducing the reproduction potential below the limit reference point is small (e.g. 5%).

1 Introduction

A framework for provision of scientific advice for the management of new and developing marine invertebrate fisheries has been developed [l].This framework explicitly endorses the precautionary approach for fisheries management and research, as advocated by Garcia [2], FAO [3] and FAO [4], due to the very nature of fisheries resources which are highly variable, poorly controllable and slowly reversible. The guidelines of this framework require a 3-phased approach in development of invertebrate fisheries. The first phase is to synthesise available biological and fisheries information on the target and sirniIar species and to identify missing information. The second phase is to obtain the essential information that is lacking from the analysis of the first phase and to evaluate alternative management strategies. The third phase is to start off a fishery based on the selected management strategies and to monitor the stock dynamics to refine the scientific information. For the past few years, there has been a major proliferation of action on the development of biological Reference Points (RPs) to achieve precautionary fishery management goals. A RP is defined as a conventional value derived from technical analysis, which is believed to be useful for the management of the unit stock [5]. FAO 161 and Caddy and Mahon [5]put forward the concept of using a pair of RPs, Limit Reference Point (LRP) and Target Reference Point (TRP), to manage fisheries stocks. A LRP indicates a state of a fishery which is considered undesirable and which management action should avoid. A TRP indicates a state of fishing which is considered to be desirable. When management action aims at TRP, there should be little risk of exceeding LRP.

Horse clams occur commonly along the west coast of North America from California to Alaska 171. They are found from the low intertidal to subtidal depths of 30-50 m [X, 71. Horse clams have been harvested subtidally in B.C. since 1979. This fishery has been restricted as an incidental by-catch to the geoduck

(Panopea abrupta) since 1992, as stock assessment was not available. Commercial harvesters of horse clams now wish to expand the fishery from an incidental to a directed fishery to more productively explore this resource. Although this is not a new fishery, the development of a directed horse clam fishery also follows the guidelines of the framework outlined above. The paper describes how we provide to the fishery managers the scientific information and advice on management strategies under the guidelines of this framework.

2 Synthesis of biological information on horse clams

Lauzier et. al. [9] reviewed biological and fisheries information on horse clams based on surveys, scientific literature, technical reports and communications with fishers. Information essential for assessing horse clam stock dynamics includes morphology, preferred habitats, distribution, population size, size and age structure, reproductive characteristics, growth, and mortality.

There are two species of horse clams in British Columbia (B.C.), Tresus capax and Tresus nuttallii, (Bivalvia: Mactridae). The two species differ morphologically in siphonal plates, shape of the shells, and internal anatomical features [10, 11, 12, 131. Despite the morphological differences, divers are generally not able to differentiate between the two species or to identify sizes, before they have been extracted from the substrate. Horse clams loose the ability to re-burrow at approximately 60-75 mm shell length [14]. Thus, the fishery is a mixed species fishery and recruitment size or age to the fishery can not be easily regulated. Horse clams live in mud, sand, gravel, and shell substrates along B.C. coastal waters [15, 7, 161. T. nuttallii is found from the low intertidal to subtidal depths of 50 m, buried to depth of, at least, 1 m, whereas T. capax is found from rnid- intertidal to subtidal depths of 30 m, but buried not as deep as T. nuttallii [S, 71. There is a considerable geographic variation in the species composition of the two horse clam species. In some horse clam beds in B.C., one species is overwhelmingly dominant, while in others, there is a varying degree of mixture 117, 181. The density of horse clams varies considerably in different beds. Horse clam surveys showed that the mean density was 0.32/m2 in the Ritchie Bay [18],

1.48/m2in the Puget Sound [19], and 4.54/m2in Seal Island [IS]. Productive horse clam beds are often closely associated with extensive distribution of eelgrass (Zoostera marina), which is an important habitat for herring spawning. Horse clam fishing has been restricted to 10 feet below char datum in an attempt to protect eelgrass beds. Subtidally, there is an extensively overlapped distribution of horse clams and geoducks [20]. Consequently, the effect of horse clam harvesting on geoduck population should be considered. Due to lack of appropriate stock assessments, managers have restricted exploitation of horse clams as only incidental by-catch to the geoduck in B.C. This incidental fishery, since 1992, has been a 3-year rotation fishery, CO-incidentwith the geoduck fishery management plan. Arbitrary and conservative catch ceilings have been imposed loosely based on historical catches. Average annual landing of horse calms in B.C. was 131 tonnes for the period between 1979 and 1991 and was 13 tonnes for the period between 1992 and 2000. A 3-year rotation fishery is possibly less detrimental than an annual one to the geoduck population, and should be maintained for the directed fishery. Horse clams can be aged by counting annuli on the external surface of the shell [13, 211. Among the samples collected from studies, the maximum shell length was 230 mm for T. nuttallii and 187 mm for T. capax; the maximum age was 24 years old for T. nuttallii and 21 years old for T. capax 1181. Few horse clams younger than 3 years old were collected, as divers could hardly see them. Horse clams reach sexual maturity at a shell length of 70 mm for both species [13, 221 and at age of 3-4 years for T. capax and 3 years for T. nuttallii [23]. The spawning season for T. capax is between late February and early May [13]. T. nuttallii mainly spawns around summer [24]. Horse clams grow quickly after settlement. T. capax grows by 22-25 dyear in the first 3 years 113, 251. Thereafter, growth declines gradually with age.

Campbell et al. [23] and Campbell and Bourne [l81 described the growth of T. nuttallii and T. capax in B.C. using the von Bertalanffy model. They not only estimated the three parameters of this model, L, k, and to, but the associated 95% confidence intervals for each parameter estimate. Campbell et al. [23] and Campbell and Bourne [l81 described the length (L) and weight (W) relationship using the model: ln(W) = InA + B * ln(L), where InA and B are model parameters.

Total mortality rates were estimated for adult T. nuttallii and T. capax by calculating the slope of the regression line on the descending right limb of the age frequency curves [IS]. Associated 95% confidence intervals were also estimated. As no or little commercial exploitation had occurred in the areas from which these samples were collected, these estimates were regarded to be the estimates of natural mortality rates (M). M varies from 0.2 to 0.4 for T. nuttallii depending on the location. M was estimated to be 0.16 for the subtidal T. capax in Seal Islets. Lauzier et. al. [9] identified some lacking biological information, such as the relationship between stock size and recruitment. The most important task is, however, to conduct modelling to investigate fishing strategies and set up appropriate RPs.

3 Evaluation of management strategies

To assess the effect of alternative fishing strategies and set up RPs, we need an appropriate model which is able to fully use the synthesised information. Biomass production model, spawning and recruitment model, yield per-recruit model (YPR), or spawning stock biomass per-recruit (SPR) is commonly used for assessing stock dynamics. Fitting the biomass production model requires either a time series of catches and relative abundance indices (such as catch per unit effort) or information on growth, mortality and stock and recruitment relationship [26, 271. No reliable time series of effort data or stock-recruitment relationship were available for horse clam populations in B.C. We do, however, have information on growth, MS, age of maturity, and longevity of horse clams.

This allows analysis of YPR or SPR. YPR model does not explicitly consider conservation of the reproductive resource. SPR model is likely to be superior, as it takes account of maturity schedules in addition to the process captured by YPR analysis [28].

To incorporate uncertainties about estimates of population parameters into the study, a stochastic SPR model was developed [29]. Conventionally, SPR model is fitted in a deterministic way, as though the estimates of biological parameters are perfect without any error or uncertainty. There is a one-to-one relationship between percentage of maximum SPR and exploitation rate, after the age of recruitment to the fishery is determined. In reality, however, our knowledge about the status of the fish populations almost always has inherent uncertainties. Caddy [30] recommended that population model development should take into account the imprecision of population parameters. Estimates of natural mortality

rates and parameters of the von Bertalanffy model for horse clams were all associated with uncertainties expressed as 95% confidence intervals. Variations in the weight for a given length could also be estimated. Due to these uncertainties, one level of exploitation rate might lead to a variety of possible consequences in terms of reproduction potential, which is measured as the percentage of maximum SRP (achieved when with no fishing) retained by the stock. The probability of each possible consequence could be estimated through this simulation study, allowing the fishery managers to examine the various possible consequences. Maximum age was estimated to be 24 years old for T. nuttallii and 21 years old for T. capax by counting the annuli on the shells. As annuli formed on the margin of shells of old horse clams might not have been entirely enumerated, and older horse clams might not have been sampled, Maximum age was conservatively set to be 30 years for both T. nuttallii and T. capax in the model.

As recruitment size or age of horse clams to the fishery cannot be easily regulated, it was determined by analysis of age frequency distributions produced by Campbell and Bourne [18]. This revealed that horse clams begin to be vulnerable to harvesting approximately at age of 3-4 years, when they just reach maturity. The 3-year rotation harvesting scheme is likely to be maintained, which not only reduces possible detrimental impact on geoduck population, but allows depleted local populations to have a better chance to recover before re- harvesting. In addition, fishing activities are concentrated and easily monitored in a rotation fishery. However, the fishery managers might like to know the effect of other fishing schemes as well. In the simulation study, three fishing schemes, annual, Zyear and 3-year rotation fishery, were investigated. M appears to be quite variable depending on areas and species of horse clams. Nine mean possible natural mortality rates (01 - 0.5 with an interval of 0.05) were used in the simulation model. Coefficient of variation is set to be 0.3 based on the average of the estimated variations.

A n-year rotation fishery (n = 2, 3 ...n) has a fishing cycle of one year of fishing followed by n-l years of non-fishing for any specific area or bed. Unlike an annual fishery where recruits from any year experience the same amount of fishing pressure, recruits from different years of a fishing cycle of a rotation fishery go through different fishing patterns in their whole life span. An average recruit from different years of a fishing cycle produces different amounts of spawning stock biomass in the entire lifespan. For a n-year rotation fishery, there are n different SPRs corresponding to n different years of a fishing cycle. The impact of fishing on the reproduction potential was measured simply by calculating the average of the SPRs. As divers are generally not able to assess individual size during harvesting, the actual exploitation rate (E) on each age class is presumably variable, even though a fixed E is applied to the whole population. Coefficient of variation for E is arbitrarily set to be 0.1 (one third of the coefficient of variation for M). Effect

of 40 Es (0.01 to 0.4, with an interval of 0.01) on the reproduction potential was investigated. One thousand simulations were carried out for the combination of each of the

3 fishing schemes, each of the 9 mean MS and each of the 40 Es for each species. There are, therefore, 1080 ( 3~9x40)combinations for each species of horse clam in the simulation study.

Each simulation was run with values for growth, M, and E on each age class generated randomly with estimated or assumed variations around each of the mean values to produce a possible consequence in terms of reproduction potential. Thus, 1000 simulations produce a distribution of 1000 possible consequences. The fishery managers could find from the probability distributions produced by the simulation model such an E that the probability of reducing the reproduction potential below the pre-set value of reproduction potential is not more than the pre-set risk [29].

To choose a proper E, we need know what percentage of maximum SPR the stock should retain in order to have a viable and sustainable fishery. After analysing spawning-recruitment data together with SPR modelling, Mace and Sissenwine [31] suggested that the percentage of the maximum SPR, at which successive generations can just, on average, replace each other without surplus production, should be assumed to be 30%, when there is no other means to estimate it. Based on simulations on stock-recruitment relations of a variety of groundfish, Clark [33] proposed that a TRP can be set to F40%,a fishing rate which will produce 40% maximum SPR. Mace [28] also recommend to use F40% as a TRP for fin-fish, when stock and recruitment relationship is unknown. Compared with fish-fish, sedentary invertebrates, such as horse clams, are almost immobile. Harvesting of sedentary invertebrate populations tends to be in a 'pulse' fishing mode, in which local patches may be removed selectively, and the effect of past local harvesting remain local, at least until the population re- generates [30]. The application of SPR relies on the existence of density dependence on population renewal. The impact of reduced density in patches of horse clams on reproduction capacity is not known. Thus, RPs were recommended to be set in a highly precautionary manner at the initial phase of developing this fishery.

Caddy and McGarvey [32] suggest that the LRP should be set first. Choosing a LRP before a TRP leaves open how the TRP might be calculated. Because LRPs are to be avoided, TRP for exploitation rate has to be set considerably lower than the LRP, so that the probability of exceeding the LRP is very low.

LRP was recommended to be set to 70% maximum SPR at the initial stage of developing a directed horse clam fishery. E should be chosen from the probability distribution of possible reproduction potentials to be such a low value, that the probability of exceeding the LRP will be only 5%. The stock status should be monitored and the fishing strategy is to be re-evaluated when new information becomes available. In most subtidal horse clam beds, T. nuttallii and T. capax CO-existwith a varying degree of mixture. When there is a good

mixture of the two species, the lower E should be used to avoid possible overfishing of either species.

4 Discussion

Traditionally, a new fishery often proceeds without sufficient amount of knowledge on the population biology. When the population is fished down and profits are increasing, more and more fishermen are attracted to the fishery. Catches will eventually fall at high fishing effort, as the population production can not compensate for the fishery removals. Usually at this "crisis" stage, the fishery managers start to seek management actions [l]. However, the unsustainable stock may continue to be fished at a high level of fishing effort due to economic or social reasons. The stock faces a risk of collapsing. To ensure precautionary and sustainable use of natural resources, Perry et al.

[l] put forward a framework for development of a new invertebrate fishery. Under the guidelines of this framework, a new fishery only proceeds after basic biological information on the population has been gathered, stock dynamics has been assessed, and management strategies have been identified. Development of a directed horse clam follows the guidelines of this framework. Biological and fisheries information was synthesised based on scientific papers, technical reports and surveys. The synthesis provided knowledge on many important biological aspects, such as growth, natural mortality, longevity, reproductive characters, and age of vulnerability to fishing. These biological features determine, to a large extend, the dynamics of horse clam population and were used through a simulation model to evaluate management strategies and set up biological RPs.

The simulation model was developed to fully utilise the available information. Several measures were taken to be precautionary in assessing the stock dynamics. The model incorporates variations in estimates of growth and MS, converting the uncertainties inherent in scientific knowledge into a probability distribution of consequences in terms of reproduction potential. Higher uncertainties would result in higher variance in the consequences. The fishery managers can set up the level of E not only based on the knowledge on the estimates of these biological parameters, but also, implicitly, on the degree of uncertainties associated with these estimates. The longevity (maximum age) of horse clams was assumed to be higher than estimated. A population of higher longevity is generally more vulnerable to exploitation, so that it should be explored with a relatively low E. Clark and Mace proposed that a TRP be set to 40% [33] [28] maximum SPR for the fin-fish. TRP was recommended to be set much more conservatively at the initial stage of developing a directed horse clam fishery. A LRP was recommended to be set to 70% maximum SPR. A TRP of E was recommended to be set so low that that the risk of reducing the reproduction potential below the LRP should not be more than 5%. The most important lacking information is the relationship between stock biomass and recruitment, which requires many years of study and data collections. When the information is known, simulations based on a yield model,

as described by Constable et al. [34], can be used to determine the precautionary harvest strategy which has a pre-set small chance to reduce the spawning biomass and recruitment below a pre-set level.

References

Perry, R.I., Walters, C.J., & Boutillier, J.A. A framework for providing scientific advice for the management of new and developing invertebrate fisheries. Rev. Fish Biol. Fish., 9, pp. 125-150, 1999. Garcia, S.M. The precautionary principal: its implications in capture fisheries

management. Ocean & Coastal Management, 22, pp. 99-125,1994. FAO. The Precautionary approach to fisheries with reference to straddling fish stock and highly migratory fish stocks. FAO Fish. Circ., 871, 1995.

FAO. Precautionary approach to capture fisheries and species introduction. FAO Tech. Guidelines for Res. Fish., 2, 1996. Caddy, J.F., & Mahon, R. Reference points for fisheries management. FAO Fish. Tech. Rap., pp. 347-383, 1995.

FAO. Reference points for fisheries management: their potential application to straddling and highly migratory fish stocks. FAO Fish. Circ., 864, 1995. Bernard, F.R. Catalogue of the living Bivalvia of the eastern Pacific Ocean: Bering Strait to Cape Horn. Can. Special Publ. Fish. Aquat. Sci., 61, 1983.

Haderlie, E.C. & Abbott, D.P. Bivalvia: The clams and allies. Intertidal invertebrates of California, eds. R.H. Morris, D.P. Abbott & E.C. Haderlie, Stanford Univ. Press, Standford, California, pp. 355-41 1, 1980. Lauzier, R.B., Hand, C.M., Carnpbell, A., & S. Heizer. A review of the

biology and fisheries of horse clams (Tresus capax and Tresus nuttallii). Can. Stock Assess. Secretariat Res. Doc., 98/88, 1998. [l01 Swan, E.F., & Finucane, J.H. Observations on the genus Schizothaerus.

Nautilus. 66, pp. 19-26, 1952. [l11 Pearce, J.B. On the distribution of Tresus nuttallii and Tresus capax ((Pelecypoda: Mactridae) in the waters of Puget Sound and San Juan Archipelago. Veliger, 7, pp. 166-170, 1965.

[l21 Stout, W.E. Some associates of Tresus nuttallii (Conrad, 1837) (Pelecypoda: Mactridae). Veliger 13, pp. 67-70, 1970. [l31 Bourne, N., & Smith, D.W. Breeding and growth of the horse clam, Tresus

capax (Gould) in southern British Columbia. Proc. Nat. Shellfish Assoc., 62, pp. 38-46, 1972. [l41 Pohlo, R. H. Ontogenetic changes of form and mode of life in Tresus nuttallii (Bivalvia: Mactridae). Malacologia, 1, pp. 321-330, 1964.

[l51 Quayle, D. B. The intertidal bivalves of British Columbia. B.C. Proc. Museum Handbook, 17,1960. [l61 Coan, E. V., Scott, P. H., & Bernard, F. R. Bivalve seashells of western North America. Santa Barbara Museum of Natural History Publication,

Viii, 2000.

[l71 Bourne, N.F., & Harbo, R. Horse clams. Status of invertebratefisheries of the Pacific coast of Canada (1985/86), eds. R.M & G.S. Jamieson, Can. Tech. Rep. of Fish. Aquat. Sci., 1576, pp. 89-94, 1987.

[l 81 Campbell, A., & Bourne, N. Population biology of the gaper (horse) clams, Tresus capax and Tresus nuttallii, in southern British Columbia. J. Shellfish Res., 2, pp. 933-942,2000. [l91 Goodwin, L. & Shaul, W. Puget sound subtidal hard shell clam survey data.

State of Washington, Department of Fisheries, Progress Report No. 44, 1978. [20] Goodwin, L. & Pease, B. The distribution of geoduck (Panopea abrupta)

size, density, and quality in relation to habitat characteristics such as geographic area, water depth, sediment type, and associated flora and fauna in Puget Sound, Washington. Wash. Dep. Fish. Tech. Rap., 102, 1987. [21] Wendell, F., DeMartini, J.D., Dinnel, P., & Siecke, J. The ecology of the

gaper or horse clam, Tresus capax (Gould 1985) (Bivalvia:Mactridae), in Humboldt Bay, California. Calif. Fish and Game, 62, pp. 41-64, 1976. [22] Clark, P.C., Nybaken, J. & Laurent, L. Aspects of the life history of Tresus nuttallii in Elkhorn Slough. Calif. Fish and Game, 61, pp. 215-227, 1975.

[23] Campbell, A., Bourne, N., & Carolsfeld, W. Growth and size at maturity of the Pacific gaper Tresus nuttallii (Conrad 1837) in southern British Columbia. 5. Shellfish Res., 9, pp. 273-278, 1990.

[24] Quayle, D. B., & Bourne, N. The clam fisheries of British Columbia. Bull. Fish. Res. Board Can., 179, 1972. [25] Breed-Wilke, G.M., & Hancock, D.R. Growth and reproduction of subtidal and intertidal populations of the gaper clam Tresus capax (Gould) from

Yaquinna Bay, Oregon. Proc. Nat. Shellfish Assoc., 70, pp. 1-13, 1980. [26] Shepherd, J.G. A versatile new stock-recruitment relationship of fisheries and construction of sustainable yield curves. Cons. Perm. Int. Explor. Mer., 40, pp. 67-75, 1982.

[27] Sissenwine, M.P., & Shepherd, J.G. An alternative perspective on recruitment overfishing and biological reference points. Can. J. Fish. Aquat. Sci. 44,913-918, 1987.

[28] Mace, P.M. Relationships between Common Biological Reference Points Used as Thresholds and Targets of Fisheries Management Strategies. Can. J. Fish. Aquat. Sci., 51, pp. 110-122, 1994. [29] Zhang, Z., & Campbell, A. Application of a Stochastic Spawning Stock

Biomass per Recruit Model for the Horse Clam Fishery in British Columbia. Fish. Res., 57, pp. 9-23, 2002. [30] Caddy, J.F. Background concepts for a rotating harvesting strategy with particular reference to the Mediterranean red coral, Corallium rubrum.

Mar. Fish. Rev., 55, pp. 10-18, 1993. [31] Mace, P.M., & Sissenwine, M. How much spawning per recruit is enough? Risk evaluation and biological reference points for fisheries management,

& eds. S.J. Smith, J.J. Hunt, D. Rivard, Can. Spec. Publ. Fish. Aquat. Sci., 120, pp. 101-118, 1993.

[32] Caddy, J.F., & McGarvey, R. Targets or limits for management of fisheries? North Am. J. of Fish. Man., 16, pp. 479-487, 1996. [33] Clark, W.G. The effect of recruitment variability on the choice of a target

level of spawning biomass per recruit. Proc. Intl. Symp. Management Strategies for Exploited Fish Populations, Alaska Sea Grant College Program: AK-SG-93-02, pp. 233-246, 1993. [34] Constable, A.J., de la Mare, W.K., Agnew, D.J., Everson, & Miller, D. I., Managing fisheries to conserve the Antarctic marine ecosystem: practical implementation of the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR). ICES J. Mar. Sci., 57, pp. 778-791,2000.