b Gear selectivity in the directed cod fishery (BACOMA project)

Transcription

1 b Gear selectivity in the directed cod fishery (BACOMA project) IBSFC has asked ICES to: i) evaluate the potential improvement in the gear selectivity in the directed cod fisheries as concluded by the Project for Improvement of Baltic Cod Management (BACOMA) and estimate effects of changes in the exploitation pattern on the cod stocks and the fisheries Mesh size selection is a strategic, long-term decision in which it is not possible to assess the exact consequences of choices in terms of future catches or biomasses. Mesh size rules and minimum landing size rules are used to improve the utilisation of the growth potential of the stock, or to reduce the exploitation of that part of the stock, which has not yet contributed to reproduction (fish under the maturity length). ICES has previously advised on minimum mesh size in 1993 when ICES proposed that a 120 mm diamond mesh be introduced, based on 25 = 38 cm. IBSFC introduced the proposed minimum mesh size (120 mm) together with two designs of exit windows in a 105 mm trawl. These windows were termed the "Danish" and "Swedish" windows. In practice, fishermen used the Danish window; it has a lower selectivity pattern and therefore higher catch per hour in the present situation. The use of the old "Danish" window was banned in 1999 and replaced with a "New Danish" window in a 105 mm codend. The minimum landing size (MS) was set at 35 cm with 5% allowance for undersized cod in the catch in weight, IBSFC Fishing Rule 8. The BACOMA results refer to a trawl with an exit panel (the BACOMA window) inserted in the uppermost parts of the cod-ends. The size of the exit window is increased in the aftmost parts of the codend compared to designs previously used. This gear has similar selectivity properties as other trawls with exit windows now in the IBSFC fishing rules. The mesh size referred to in the simulations is the mesh size in the exit window of BACOMA type. Baltic cod are harvested by both trawl and by gill nets. The BACOMA project investigated selectivity in trawl fisheries and survival of fish passing through the meshes in the trawl. There are no new data on the selectivity of gill nets. The BACOMA experiments were done in areas with eastern Baltic cod which have somewhat different growth and mortality rates compared to western Baltic cod. The presented simulation studies are based on data for the eastern Baltic cod stock and the results may therefore not be totally applicable to the combined stock. ICES notes that IBSFC will establish a sub-group that will focus on the technical details of the trawl types to be implemented in future. ICES stresses that these details are important in order to ensure that the chosen trawl selectivity will be realized properly. Objectives and evaluation criteria ICES focused its evaluation of effects of different mesh sizes on impacts on catches, size of fish in the catch, total yield from the stock, spawning biomass, capture rate of immature fish, and catch rates of fish below legal landing sizes. Managers have given no specific objectives for these properties, nor clear ranking of importance among them. Hence, ICES lacked explicit criteria which could be used to find an optimal mesh size. Management bodies also must find a balance between the long-term gains and short-term losses resulting from mesh size increases. Again, there are no specific guidelines for weighing costs and benefits. Because discarding has a direct negative impact on the future catch potential, ICES advises that mesh size and the minimum landing size (MS) be chosen in balance in order to reduce discarding. Survival of cod that escape from the cod-end in trawls The BACOMA results indicate clearly that the mortality of Baltic cod that escape from the cod-end is generally very low (on average less than 3%). The high survival of escapees enables the effective use of mesh size as a stock conservation management tool. Fleets, selection and discards To the extent possible, BACOMA has estimated the selectivity properties of the commercial trawl fleet relative to present commercial fishing conditions of the whole trawl fleet, in particular to the proportion of side and stern trawlers fishing at present. The estimated selectivity parameters for the current fleet structure are given in Table b.1. The selectivity of the new 105 mm Danish window that was introduced from 1999 is not exactly known, but it retains more small cod than the mesh size 105 mm BACOMA window. All simulations assume that proportions of vessel groups will not change in the future. Y/R simulations of the long-term effects of a mesh increase in the eastern Baltic are presented in Table b.2. The simulations do not consider responses of the gill net fleet to changes in the size composition or abundance of the cod stock that would result from different meshes in the trawl gear. If the gill net fleet changes the mesh size or the proportion of gill nets in the total fleet changes, actual results will differ from the simulation results. The average 50% retention length ( 50 ) of the present Baltic cod demersal trawl fishing fleet is close to the current minimum landing size (MS = 35 cm). This means that large amounts of undersized fish are retained 66

2 by the trawls and subsequently discarded. Field sampling of the Baltic cod fishery showed that discarding of cod has been high (Table b.3), especially in western Baltic. These values are yearly mean values that depend strongly on the recruited year class.. Monthly mean values varied between 1% and 68%. Discards are minimised by setting MS in the lower range of the selectivity curve. ICES normally advises that the minimum landing size should be no greater than corresponding to 25. However, BACOMA works with 10 and in general, setting MS at 10. will give less discards than using 25. The modelling indicates that discarding rate decreases quickly when mesh size is increased (Table b.4). Stern trawlers have a lower selectivity curve than side trawlers and therefore they select more small cod than the side trawlers. Hence the selectivity of this gear is more poorly matched to the minimum landing size and to the discarding rule given the MS. Simulations (Y/R calculations) indicate that minimal discards (MS = 35 cm) are obtained for mesh sizes of 140 mm and larger (Table b.3). Impacts on stock and fisheries When there are two gear types with different selectivity properties fishing the same stock, a change in selectivity of one gear effects the catch of the other gear. When increasing the trawl mesh size, catches in gill net fishery improve more quickly than landings in the trawl fishery (Table b.4). The present average 50% retention length (32 cm) in the Baltic demersal trawl fishing fleet is substantially smaller than the length at first maturity of cod (ca. 40 cm). Hence much of the catch has never contributed to the spawning biomass. The simulations indicate that an increase in mesh size would lead to higher spawning stock biomass, while fishing mortality is kept at F=0.6, the agreed level for the Eastern Baltic Cod Stock. The direct increases in SSB from larger meshes also effect future recruitment. The estimation of the contribution of larger SSB to recruitment depends crucially on the stock recruit model used, and on assumptions about cannibalism. For the eastern Baltic cod, effects of inflow of saline water to the Baltic Sea also need to be considered in recruitment predictions. Simulations were performed using both a Ricker stockrecruitment model and an Environmental model including the saline inflow dynamics. The environmental model assumed similar short-term trends in the production success of a tonne of spawning biomass as observed in the historical data set for spawning volumes. The simulation results are summarised in Table b.5. Using the environmental model, an increased mesh size would create a reserve in the spawning capacity, which would utilise the improved water quality when an impulse of saline water takes place. The Ricker S/R model does not suggest as large an improvement as suggested by the environmental model. In this model, due to the implicitly assumed low risk of stock collapse and the cannibalism at high spawning biomass, the highest catch is obtained by mesh size 150 mm. The B&H model used for medium-term projection in section d was not used in these simulations since this model does not account for the potential improved recruitment to the fishery by avoiding discards. The models give very different perspectives on the production capacity of the stock and demonstrate that it is not possible to predict exactly the positive gains obtained by different mesh sizes, because these estimates depend on the model assumptions (Table b.5). The SSB results obtained by the Y/R model (Table b.2) are probably underestimates, because gill net fishing is likely to react by increasing the mesh size. This could also be supported by a new mesh size rule in the gill net fishery. Short-term effects An increase in mesh size causes an immediate drop in the trawl catches and an immediate increase both in gill net catches and in spawning stock biomass due to contributions from fish in length classes that are no longer retained by the trawl. Short-term losses increase with increasing mesh size. These effects are illustrated on Figure b.1. Simulations of short-term development indicate that after an initial decline, trawl landings increase for larger mesh sizes, but do not for all mesh sizes reach the present average level. The increase is partly due to reducing discards and partly from better utilisation of the growth potential of codtac or other regulations are required to avoid escalation of total effort. The analyses indicate also that the short-term losses to the fleet would be minimised if the mesh size change is made when a good year class is coming into to the fishery. However, it is impossible to predict the good year classes in a time schedule needed for mesh size change. Conclusions and further considerations Both the gains and losses from increasing mesh sizes show diminishing effects with increasing mesh sizes; i.e. effects are greater moving from 110 mm to 120 mm than from 150 mm to 160 mm (Table b.2). To avoid discards of immature cod the mesh size should be at least 130 mm. For this mesh size of 130 mm around 90 % of the cod of mean length at maturity (ca. 40 cm) are estimated to escape from the cod end. In addition, retention in the stern trawler fleet, with lower selectivity, 67

3 would be in balance with present minimum landings size ( 10 of stern trawlers with 130 mm mesh = 34 cm, Table b.1). Therefore no major discards are predicted taking into account the variability in commercial fleet selection. Increased mesh sizes would decrease discards and improve total landings. The minimum landing size (MS) should support the overall selectivity target. If the MS is not increased in tempo with an increase in the regulated mesh size, it may encourage fishermen to manipulate the selectivity characteristics of their trawls to maintain the present exploitation pattern. Therefore, a supporting minimum landing size is a necessary management action (back-up) when the minimum mesh size is increased. The absolute predicted gains vary grossly between stock-recruitment models. However, even though predicted changes depend on model assumptions, all the conducted simulations support the same conclusion: An increase in trawl mesh size would improve the status of the eastern cod stock and increase the total landings in the medium-term. A large proportion of the increase in landings would be caused by decrease or disappearance of discards. The present state of Eastern Baltic cod is very poor and the present selectivity does not allow fully utilising the current growth and recruitment potential. 68

4 Table b.1 Parameter (cm) 10% 25% 50% 75% 10% of stern trawlers The selectivity parameters estimated for the trawl fleet assuming the present proportions of stern (19 %) and side trawlers (81%). The 105 mm old Danish windows selectivity is not obtained in the BACOMA project and does not account for vessel variability. This gear was in use until mm old Danish Mesh Size n. a n. a

5 Table b.2. Y/R simulations (long-term) of SSB, yield, discards, and mean weight in catches for different mesh sizes in the commercial trawl fleet. F = 0.6 for fully recruited age-groups (=F pa ) and MS (35 cm) is assumed. The value of long-term recruitment of one year old cod is assumed to be 367 million individuals backcalculated from the long-term mean recruitment of two year old cod. The present gill net partial fishing mortality and mesh size is applied in the calculation. Windows mesh type applied Mesh size Variable 105 old Danish Spawning biomass ( 000 t) Total landings in weight ( 000 t) Trawl landings in weight ( 000 t) Gill net landings in weight ( 000 t) % of fish under the MS in all catches (in numbers) % of fish under the MS in mean catches of trawl fishing (in weight) Mean weight of fish in trawl landings (kg) Mean weight of fish in gill net landings (kg) % 13% 9% 4% 2% 1% 1% 0% 11% 6% 4% 2% 1% 0% 0% 0% Table b.3. Observed mean yearly percentages of discarded cod in numbers in different sub-divisions. Sub - division Year % 46% 15% 8% 10% 6% % 20% 8% 10% 15% 21% 70

6 Table b.4. Percentage (in weight) of fish under the MS in trawl catches calculated for eastern Baltic cod as function of BACOMA window mesh size and minimum landing size (MS). The calculations are for F = 0.60 (=F pa ) for fully recruited age groups. Estimates with the present MS = 35 cm are marked in bold. Window Mesh size MS (cm) % 2% 1% 0% 0% 0% 0% 35 6% 4% 2% 1% 0% 0% 0% 37 10% 7% 3% 1% 1% 0% 0% 39 15% 11% 5% 2% 1% 1% 1% 41 22% 17% 8% 4% 2% 2% 1% 43 31% 25% 14% 8% 6% 4% 4% 45 42% 35% 24% 17% 13% 11% 11% Table b.5. Summary of the changes in yield in the long-term, when assuming a BACOMA window codend is in use in the commercial fishing fleet. A positive effect on recruitment assumed and described by two alternative stock recruitment models (S/R model). F = 0.6 for fully recruited age-groups (=F pa ) and MS (35 cm) assumed. Only trawl mesh size is changed and present gill net fishery effort and mesh size assumed. Mesh size Model applied Yield by standard S/R model ( 000 tonnes) Yield by environmental S/R model ( 000 tonnes)

7 Spawning biomass % of present 300% 250% 200% 150% 100% 50% 0% Year Yield, gill net 400% 300% 200% 100% 0% MS =110 MS =120 MS =130 MS =140 MS =150 MS =160 Yield, trawl % of present 120% 100% 80% 60% 40% 20% MS =110 MS =120 MS =130 MS =140 MS =150 MS =160 0% Year % of present MS =110 MS =120 MS =130 MS =140 MS =150 MS =160 Year Figure b.1. Short-term effects of trawl (BACOMA window) mesh size change when different mesh sizes (MS) are applied. It is assumed that all variables are constant before year 16, when the mesh size change is assumed to take place causing the relative F at age to change. F-at-age is then constant for the rest of the period. Gill net effort and gill net mesh size are assumed to be unchanged. The results differ slightly from those presented in Table b.4 as the model used here does not include an explicit account of the length distribution by age while the Y/R model used in the table uses length distributions by age. 72