On the Joint Management of Catch and Bycatch and the Use of Marine Reserves

IIFET 2006 Portsmouth Proceedings

ON THE JOINT MANAGEMENT OF CATCH AND BYCATCH AND THE USE OF MARINE RESERVES

Siv Reithe, Norwegian College of Fishery Science, University of Tromsø, Norway. e-mail: [email protected]

ABSTRACT This paper explores the possibility of using marine reserves to protect stocks subject to bycatch problems. The importance of migration rates and growth rates of both target and bycatch species and costs are analyzed. Pure open access equilibrium harvest of target species and stock level of bycatch species are compared to those generated by a reserve and open access in the harvest zone. Win-win situations, situations where both harvest of target species and stock size of bycatch species increase, are searched for. It is shown that using a reserve to protect a slow moving bycatch species is more likely to be a success than in the case of a fast moving bycatch species. Furthermore, a fast moving species is worse off after the introduction of a reserve if it has an intrinsic growth rate that is lower than the target species, and in some cases a reserve may actually drive the bycatch stock to extinction. No win-win situations are found in the case of a fast moving species. In the case of a slow moving bycatch species a reserve increases stock size and in cases with low unit cost of effort win-win situations are identified.

INTRODUCTION Bycatch is a common problem in fisheries. [1] estimated the world’s total discards to be about 25% of the total commercial catch (by weight). In the fishery south east of Australia more than 100 species are caught, but only 15 of them account for 80% of the value [2]. In the Northeastern US only half of the total catch (by weight) taken by ground fish trawl, sink gillnet and scallop dredge vessels at Georges Bank during the 1990s consisted of the targeted species [3]. In the same region, protected species like harbor porpoise, right whale, leatherback and green sea turtles are taken as bycatch in different fisheries [4]. Other examples of fisheries with bycatch problems are dolphins caught in tuna fisheries and juvenile cod, haddock, redfish and greenland halibut taken as bycatch in the Barents Sea shrimp fishery.

In many fisheries where there are bycatch problems the problem is sought dealt with with technical constraints on gear such as sorting grids in trawl bags, mesh size and shape restrictions. Also economic instruments such as taxes or tradable permits may be used to protect species that are taken as bycatch. This paper analyzes the possibilities of using marine reserves as a tool in the management of bycatch. The possibilities of win-win situations, defined as cases where both biomass of bycatch stock and harvest of targeted species occur, are also sought for.

Some studies on the use of temporarily closed areas as a method of reducing bycatch or protect bycatch species have been conducted. [5] compare the use of temporarily closed areas and individual transferable quotas (ITQs) on bycatch of harbour porpoise in the New England multispecies gillnet fisheries and find that for a given level of bycatch ITQs are more profitable. [6] evaluate a suggested model for bycatch management using temporarily closed areas in the Barents Sea. Here, an area is closed whenever a maximum number of juvenile fish of different species are caught in the shrimp trawl nets. The decision to close is based on bioeconomic

1 IIFET 2006 Portsmouth Proceedings criteria balancing the value of lost shrimp catches resulting from a closure, and the value of future loss of catches of fish which will be taken as bycatch if the area is not closed. The maximum number of juveniles allowed in the shrimp catches will in certain cases be so high that, in practice, closure will not occur. [7] examines how the creation of a reserve of a given size affects harvest of targeted species and stock level of bycatch species under different assumptions regarding the ecological interactions between the two species. It is found that if there are no interactions between targeted and bycatch species or if bycatch species prey on bycatch species, a reserve increase the biomass of the bycatch stock, but if the two species compete, the biomass of the bycatch stock decreases a result of the reserve.

In many analyses of marine reserves it is assumed that the biomass in the area covered by a reserve will increase as the result of the reserve. [8] reviews a number of empirical studies on the effects on marine reserves and finds that this is not the case for all species. [9] finds that for three out of ten species covered by a reserve outside South Africa no increase in abundance within the reserve could be detected. Their migratory nature was described as the probable cause for these findings. Others have found that the concentration of fish of certain species is greater in the harvest zone than within the reserve [10, 11]. In these cases other explanations have been forwarded; decreased competition with target species in the fishing zone and predator- prey interactions. In this paper we consider two cases for the bycatch stock; i) the abundance of fish is higher within the reserve then in the harvest zone, ii) the distribution of does not alter as a result of the reserve and the reason is speed of migration. In the first case we set the migration coefficient low, hence, the analysis may bee seen as a comparison of two extremes, the outcome of a reserve for one fast moving and one slow moving bycatch species.

This paper extends previous works in that speed of migration, growth rate and the success of a reserve are analyzed in the setting of a bycatch species, and in that these factors are analyzed in relation to the properties of the targeted species. It is shown that the speed of motion of the bycatch species clearly matter when measuring the effect of a reserve, but also that how fast it grows relative to targeted specie and the unit cost of effort plays an important role.

The outline of the paper is as follows; first the model is described, then a comparison of the effects of a reserve on the two bycatch stocks is made. In the third section the possibilities of win-win situations are identified. Some concluding remarks finalize the paper.

THE MODEL Pre-reserve dynamics The model for the targeted species and bycatch species with migration is from [12]. This model is chosen because it has the property that pre- and post-reserve harvest is equal. The pre-reserve dynamics with harvest are given by

( 1 ) S& = r[]S(1− S) − ES

Where S is the normalized stock level, r the intrinsic growth rate and E is normalized effort so that the catchability coefficient equals r. The rent function is defined as

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( 2 ) π = prES − aE where p is a constant price per unit catch of targeted species and a is the cost per unit effort. Under open access the equilibrium rent is zero, resulting in an equilibrium stock level of the a targeted species S = = c . pr

The equilibrium stock level, found by equating (1) to zero and solving for S, is

( 3 ) S(E) =1− E

The dynamics of the bycatch stock are described by the following:

( 4 ) Z& = gZ(1− Z) − qEZ

Where Z is the normalized stock level, g is the intrinsic growth rate and q is the catchability coefficient of the bycatch function. The equilibrium stock level of bycatch stock is

q ( 5 ) Z(E) =1− E g

Post-reserve dynamics When implementing a reserve of size 0 < m < 1, S = S1 + S2, where S1 is the stock in the reserve and S2 the stock in the fishable area and S1/m and S2/(1-m) are the densities. Growth after reserve formation is assumed to equal growth prior to reserve formation. The dynamics after reserve formation then become:

⎡ ⎛ S1 S2 ⎞⎤ ( 6 ) S&1 = r⎢S1 (1− S1 − S 2 ) − γ ⎜ − ⎟⎥ ⎣ ⎝ m 1− m ⎠⎦

⎡ ⎛ S1 S 2 ⎞ S 2 ⎤ ( 7 ) S&2 = r⎢S 2 (1− S1 − S 2 ) + γ ⎜ − ⎟ − E ⎥ ⎣ ⎝ m 1− m ⎠ 1− m⎦

Where γ is the ratio of the migration coefficient over the intrinsic growth rate. Adding (6) and (7) we get

⎡ S2 ⎤ ( 8 ) S& = r⎢S(1− S) − E ⎥ ⎣ 1− m⎦

Hence, equilibrium yield from the targeted stock is H = rS(1-S) as prior to reserve formation. With open access in area 2 equilibrium stock densities as functions of effort becomes

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( 9 ) S2 = c(1− m)

m(1− c(1+ m)) −γ − (γ + (c(1+ m) −1)m)2 + 4cm2γ ( 10 ) S = 1 2m

For the bycatch stock in question we shall consider two cases. In the first case, following Reithe (2006), we assume that reserve formation does not affect the distribution of the stock, that is, post-reserve density is uniform throughout its entire distribution area. The growth is therefore only affected by reserve formation through changes in effort and post-reserve dynamics are described by

( 11 ) Z& = gZ(1− Z) − qE(m)Z(1− m)

Equilibrium stock with a reserve and open access is given by

q ( 12 ) Z(E,m) =1− E(m)(1− m) g

This approach may be a good approximation either when the species is very fast moving, or when the catchability coefficient is low. In the second case migration is explicitly modeled;

⎡ ⎛ Z1 Z 2 ⎞⎤ ( 13 ) Z&1 = g⎢Z1 (1− Z1 − Z 2 ) − λ⎜ − ⎟⎥ ⎣ ⎝ m 1− m ⎠⎦

⎡ ⎛ Z1 Z 2 ⎞⎤ Z 2 ( 14 ) Z& 2 = g⎢Z 2 (1− Z1 − Z 2 ) + λ⎜ − ⎟⎥ − qE ⎣ ⎝ m 1− m ⎠⎦ 1− m

Where λ is the ratio of the migration coefficient over the intrinsic growth rate for the bycatch species. For this set of equations (13 and 14) there is no analytical solution, hence we will have to rely on numeric simulations solely in the following analysis. Table 1 shows the parameter values used.

Table 1. Parameter values used in simulations. Parameter Value r 0.4 γ 0.1 / 0.7 g 0.15 / 0.6 λ 0.05 p 1 q 0.1 A low value for λ is chosen. Hence, the two models for the bycatch stock may serve as two extremes. In the case where migration is implicit in the model we may think of a very fast moving species and the one including migration explicitly, using a low value for λ, may be

4 IIFET 2006 Portsmouth Proceedings thought of as a slow moving species. The catchability coefficient for the bycatch species is set to 0.1, meaning that on average a fourth of the catch taken will consist of bycatch.

WHEN DO MARINE RESERVES PROTECT SPECIES TAKEN AS BYCATCH? In this section we focus on the effects of a reserve on the bycatch stock under the assumption that it is either slow or fast moving. As will be shown below, a reserve will have different effects on the bycatch stock depending on its own migration, but the effect also depend on the intrinsic growth rate of bycatch species relative to that of targeted species, the unit cost of effort and the migration rate of the targeted species. The two latter components are important in terms of how effort changes as a result of a reserve. Figure 1 shows effort levels for different sizes of marine reserves for two different unit costs of effort under the assumption of a slow moving targeted species. Figure 2 shows the same, except here the targeted species is assumed to move faster. Table 2 gives the equilibrium effort, harvest and bycatch stock for the open access case.

effort effort A B 2.5 0.6 2.25 0.5 2 1.75 0.4

1.5 0.3 1.25 reserve reserve 0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1

Figure 1. Effort levels with different sizes of the reserve when the targeted stock is assumed to be of a slow moving species. In panel A c = 0.1, in panel B c = 0.4.

Table 2. The equilibrium effort, harvest and bycatch stock for the open access case. c E S Y g = 0.15 g = 0.6 0.1 0.9 0.1 0.4 0.85 0.4 0.6 0.4 0.6 0.9

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effort A effort B 2.5 0.62 2.25 2 0.61 reserve 1.75 0.2 0.4 0.6 0.8 1 1.5 0.59 1.25 0.58

reserve 0.57 0.2 0.4 0.6 0.8 1 Figure 2. Effort levels with different sizes of the reserve when the targeted stock is assumed to be of a faster moving species. In panel A c = 0.1, in panel B c = 0.4.

It is clear from figures 1 and 2 that the lower the costs, the higher the increase in effort after reserve formation. The speed with which targeted species move at determines for what reserve sizes the increase is going to occur.

Figure 3 shows the bycatch stock levels for different sizes of reserves and two different cost levels. We see that a reserve has a very different effect on a slow moving species compared to a fast moving one, especially if costs are very low. A reserve works better for the slow moving species than for the fast moving, which is what one should expect. In the case with the low cost, however, introducing a small reserve may actually drive the bycatch stock to extinction. This is due to the tremendous increase in effort we see in figure 2 for the low cost case and a reserve of about 20%. Further, the stock does better under pure open access unless the reserve covers about 45-50% of the distribution area of the stock. For the slow moving stock, a reserve works well as stock level with a reserve is far higher than in the pure open access case.

bycatch - stock A bycatch - stock B 1 1

0.8 0.8

0.6 0.6

0.4 0.4

0.2 0.2

reserve reserve 0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1

Figure 3. Bycatch stock development for “fast” (solid line) and “slow” (dotted line) moving species. In panel A c = 0.1, in panel B c = 0.4.

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Figure 4 shows the stock levels of the bycatch species with different reserve sizes when targeted stock is assumed to be of a faster moving species. We here see that in the very low cost case (panel A) both species are better off with a reserve than without even if effort increases just as much as in figure 3, where the target species was assumed to move slowly. The explanation for this is that the increase in effort occurs for higher reserve sizes and with a large reserve the fraction of the stock taken as bycatch available in the harvest zone is small (se figures 1 and 2).

bycatch - stock A bycatch - stock B 1 1

0.8 0.8

0.6 0.6

0.4 0.4

0.2 0.2

reserve reserve 0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1

Figure 4. Bycatch species stock levels at different reserves sizes when target species is fast moving. In panel A c = 0.1, in panel B c = 0.4.

When the growth of the bycatch species is high relative to that of the targeted species, the impact of effort changes is less severe and the difference in stock levels between the fast and slow moving species decreases.

bycatch - stock A bycatch - stock B 1 1

0.8 0.8

0.6 0.6

0.4 0.4

0.2 0.2

reserve reserve 0.2 0.4 0.6 0.8 1 0.2 0.4 0.6 0.8 1

Figure 5. Bycatch species stock levels at different reserves sizes when intrinsic growth rate of bycatch species is higher than that of targeted species. In panel A c = 0.1, in panel B c = 0.4.

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ARE WIN-WIN SITUATIONS POSSIBLE? A win-win situation is here defined as a situation when both catch of targeted species and stock of bycatch stock increase. Identification of win-win situations is important as they may ease the implementation of a reserve. Figures 6, 7 and 8 shows the difference between harvest and stock levels of slow and fast bycatch species at given reserve sizes and the pure open access harvest under different assumptions regarding costs, speed of migration for target species and the intrinsic growth rate of the bycatch species. A picture repeated in figures 6, 7 and 8 is that win- win situations with a fast moving species seem impossible. The reason for this is that when the bycatch species moves fast and there is no stock build up in the reserve, the bycatch stock will be proportional to effort. If effort increases as a result of a reserve, the potential for an increase in harvest of target species increases, but the stock of the bycatch species will decrease.

A 0.4 B 0.5 0.25 0.3 reserve 0 0.2 0.4 0.6 0.8 1 0.2 -0.25 -0.5 0.1

-0.75 reserve 0 0.2 0.4 0.6 0.8 1 -1

Figure 6. Difference in catch of targeted species (solid line) and stock levels of fast (dotted line) and slow (broken line) bycatch species when bycatch species has a low intrinsic growth rate and targeted species is slow moving. In panel A c = 0.1, in panel B c = 0.4.

Another repeated picture from figures 6, 7 and 8 is that the possibilities of a win-win situation decreases with increasing unit cost of effort (comparing panels A and B). This is because if costs are low the stock under open access is low, hence the harvest is low, and the cost associated with closing an area is low, a result in accordance with other works (see e.g. [13]). Comparing the differences in pre and post reserve stock sizes for the slow moving bycatch species in figures 6 and 7 we see that there is a smaller benefit from a reserve if the intrinsic growth rate of the bycatch stock is higher than that of the targeted species. This is because in this case the stock is better off in the pure open access case compared to when the stock has a low intrinsic growth rate; hence there is less to gain by the introduction of a reserve.

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A B

0.1 0.075 0.05 reserve 0.2 0.4 0.6 0.8 1 0.025 reserve -0.1 0.2 0.4 0.6 0.8 1 -0.025 -0.2 -0.05 -0.075

Figure 7. Difference in catch of targeted species (solid line) and stock levels of fast (dotted line) and slow (broken line) bycatch species when bycatch species has a high intrinsic growth rate and targeted species is slow moving. In panel A c = 0.1, in panel B c = 0.4.

Comparing figure 7 and 8 we see that the possible win-win situations occur for lower reserve sizes when targeted species is slow moving compared to when it is faster moving. This is because with a higher migration rate the value of a large reserve increases.

0.1 A B 0.06 0.05 Reserve 0.2 0.4 0.6 0.8 1 0.04 -0.05 -0.1 0.02 -0.15 -0.2 Reserve -0.25 0.2 0.4 0.6 0.8 1

Figure 8. Difference in catch of targeted species (solid line) and stock levels of fast (dotted line) and slow (broken line) bycatch species when bycatch species has a high intrinsic growth rate and targeted species is fast moving. In panel A c = 0.1, in panel B c = 0.4.

CONCLUDING REMARKS Comparing harvest of targeted species and stock size of a slow and a fast moving stock taken as bycatch under pure open access and the case of a reserve and open access in the harvest zone, it has been shown that the speed at which the bycatch stock moves is important to both the question of whether a reserve will protect the bycatch stock and if there are possible win-win situations.

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For slow moving bycatch species, a reserve in is better of with a reserve in all cases analyzed here. In addition, there is a range of reserve sizes that would result in win-win situations in the case of a low unit cost of effort. In the case of a fast moving species the results indicate the opposite, the bycatch stock is worse off with a reserve and in cases where the intrinsic growth rate is lower than that of the targeted species and unit cost of effort is low the introduction of a reserve may actually drive the bycatch stock to extinction.

This has implications for management. Many places where bycatch is a problem, the bycatch will consist of several species. If a reserve is implemented on the basis of protecting one species, one may accidentally worsen the conditions for another stock.

Win-win situations are important to identify because if they apply, it may ease the implementation of a reserve. If they however prove impossible to realize, this is not sufficient to reject the idea of using a reserve. Bycatch control will not be for free in most cases and if alternative bycatch control strategies are considered, the costs associated with them have to be compared along with the benefits

REFERENCES [1] Alverson , D.L., M.H. Freeberg, S.A. Murawski & J.G. Pope. A Global Assessment of Fisheries Bycatch and Discards. Food and Agriculture Organization Fisheries Technical Paper, 339, Rome, Italy, 1994. [2] Pascoe, S. 2000. Single species conservation in a multispecies fishery: the case of the Australian eastern gemfish. Ecological Economics, 32, pp. 125-136. [3] Edwards, S.F., B. Rountree, J.W. Walden & D.D. Sheehan. An inquiry into ecosystem-based management of fisheries resources on Georges Bank. In Nishida, T., P.J. Kaiola & C.E. Hollingsworth (Eds.): Proceedings of the First International Symposium on Geographic Information Systems (GIS) in Fishery Science. Fishery GIS Research Group, Saitama, Japan, pp. 202-214, 2001. [4] NMFS (National Marine Fisheries Service). 1998. Managing the Nation’s Bycatch. National Oceanic and Atmospheric Administration, US Department of Comerce, Washington, DC. [5] Bisach, K. & J. Sutinen. 2002. Reducing harbor porpoise bycatch:ITQs or time area closures? An empirical analysis of harbor porpoise in New England multi-species sink gillnet fishery. Paper presented at the IIFET conference, Wellington, New Zealand. [6] Reithe, S. & M. Aschan. 2004. Bioeconomic analysis of by-catch of juvenile fish in the shrimp fisheries - an analysis of management procedures in the Barents Sea. Environmental and Resource Economics, 28, pp. 55-72. [7] Reithe, S. 2006. Marine reserves as a measure to control bycatch problems: The importance of multispecies interactions. Natural Resource Modeling, 19(2), pp.221-241. [8] Ward, T.J., D. Heinemann and N. Evans. 2001. The role of marine reserve as fisheries management tools. A review of concepts, evidence and international experience. Bureau of Rural Sciences, Australia. [9] Bennet, B.A. & C.G. Attwood. 1991. Evidence of recovery of a surf-zone fish assemblage following the establishment of a marine reserve on the southern coast of South Africa. Marine Ecology Progress Series, 75(2), pp. 173-181. [10] Roberts, C. & N.V.C. Polunin. 1991. Are reserves effective in management of reef fisheries? Reviews in Fish Biology and Fisheries, 1, pp. 65-91.

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[11] Macpherson, E., F. Biagi, P. Francour, A. Garcia-Rubies, J. Hermelin, M. Hermelin-Vivien, J.Y. Jouvenel, S. Planes, L. Vigliola & L. Tunesi. 1997. Mortality of juvenile fishesof the genus Diblodus in protected and unprotected areas in the western Mediterranean Sea. Marine Ecology Progress Series, 160, pp.135-147. [12] Flaaten, O. & E. Mjølhus. 2005. Using reserves to protect fish and wildlife – simplified modeling approaches. Natural Resource Modeling, 18(2), pp.157-182. [13] Sanchirico, J.N. & J.E. Wilen. 2001. A Bioeconomic Model of Marine Reserve Creation. Journal of Environmental Economics and Management, 42, pp. 257-276.