114

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 175: 114-120. 1979.

A COMPARISON OF EXPLOITED POPULATIONS OF THE SCALLOP, PEC TEN MAXIMUS (L.)

James Mason, M. D. N icholson1, and A ileen M. Shanks Marine Laboratory P.O. Box 101, Aberdeen AB9 8DB, Scotland

This paper presents an examination of three scallop fisheries on the west coast of Scotland. The histories of the fisheries are reviewed, and landings, effort and catch-rate data are given for a number of years. The characteristics of two of the fisheries which are longstanding are compared with those of the third which is more recent in origin. The results of virtual population analyses are presented, giving estimates of fishing mortality rates in the three fisheries. The consequences of these estimated levels of exploitation are examined in terms of yield per recruit and biomass per recruit, and conclusions are drawn referring to possible changes in fishing policy in the three areas. The approach throughout has been to utilize methods of fish population dynamics which have not been generally applied to the assessment of shellfish stocks.

INTRODUCTION one ground as long as catch rates remain high and A scallop fishery using toothed dredges started in then moving to new ones. Grounds are revisited as the Clyde Sea area (Fig. 68) in the 1930s, operated the concentrations recover through new settlement mainly by boats less than 40 ft long towing two or and growth. Several grounds are therefore included three dredges of varying widths. The fishery remained in each of the three areas referred to here. small for some years after the second world war, The scallop’s growth slows down in the autum n providing a useful seasonal (autumn to spring) income and stops in the winter. An annual growth ring is for a few fishermen. In the 10 years 1951-1960 laid down in the spring when growth recommences landings averaged 125 tonnes annually. With the (Mason, 1957). All material has therefore been re­ advent of processing facilities markets were established lated to a “scallop year” 1 April to 31 March. for frozen scallop meat and fishing became possible Age and length composition data were obtained by all the year round (Mason, 1972). This resulted in sampling from commercial vessels. Virtually all scal­ the 1960s in an increase in fishing, at first slow and lops caught by commercial dredges are landed. Com­ later rapid, and an extension of the fishery as grounds mercial statistics provide total landings and total were discovered west of Kintyre (Sound of Jura, number of hours’ fishing but take no account of the Gigha, Islay (Fig. 68)) and later further afield. More number of dredges used. So, daily estimates of catch and bigger boats became involved, towing usually per unit effort were obtained from selected vessels for four but occasionally up to eight or ten dredges. The which information was collected on the number of dredges are now a standard width of 4 ft (1-2 m). hours’ fishing and number of dredges used. The This paper compares the population dynamics of estimates are expressed as numbers of scallops per the long-established fisheries on the east and west unit of fishing time per unit of dredge width. sides of the Clyde and the new fishery west of Kintyre. CATCH PER UNIT EFFORT AND STOCK MATERIAL AND METHODS COMPOSITION The scallop stocks consist of discrete concentrations. EAST AND WEST SIDES OF THE CLYDE In common with many bivalve molluscs, settlement The total catches of scallops on both sides of the tends to be sporadic and sometimes patchy. Fisher­ Clyde (Table 42a, b) have fluctuated since the initial men shift from place to place, continuing to fish on increase in the mid-1960s, partly as a result of a new

1 Present address: North West Water Authority, Great San- fishery for queens (Chlamys opercularis) which started key, Warrington, England. in the late 1960s and partly because boats have A comparison of exploited populations of the scallop, Pecten maximus (L.) 115

.Gierha,

55° 30 ' Ayr

$ 3

Figure 68. Southwest Scotland, showing the location of scallop grounds referred to in the text. switched grounds. They have, however, generally previously been virtually unfished, but as the fishery been higher than in the 1950s, with most fishing on developed it fell to half its original value by 1971— the west side. 1972, but it has since risen (Table 42c). It is now Catch per unit effort (c.p.u.e.) fell on both sides similar to the recently improved values for the east of the Clyde during the mid- and late 1960s (Mason side of the Clyde. and Drinkwater, 1969; 1974). This was thought to At first the landings contained a very high pro­ reflect poor recruitment, since very few scallops with portion of older scallops, 75% having seven or more fewer than six growth rings were present in commer­ rings. This proportion gradually fell as older scallops cial catches. C.p.u.e. later rose on both sides of the were removed and the age composition now closely Clyde as scallops with three, four and five rings became resembles that within the Clyde. more abundant (Mason and Drinkwater, 1975; 1976). The increase in c.p.u.e. since 1971-1972 coincided At this time 1-, 2- and 3-ring scallops were also with good recruitment, 2-, 3- and 4-ring scallops abundant in research ship catches in dredges with a having been more abundant in commercial landings, fine mesh lining. and 1-, 2- and 3-ring scallops have been evident in small-mesh research dredge catches. WEST OF KINTYRE The total catch of scallops west of Kintyre rose rapidly from the mid-1960s to a peak of 1918 tonnes POPULATION ASSESSMENTS in 1968-1969, then fell as the emphasis switched to From total landings and age composition data ob­ queen fishing but rose steadily again during 1972— tained by sampling, estimates of total landings by age 1975. C.p.u.e. and catch composition data are avail­ were derived for the years 1963-1964 to 1973-1974 able from 1966-1967, when fishing first became in­ for the west side of the Clyde, 1963-1964 to 1972-1973 tensive. C.p.u.e. was at first high since the stocks had for the east side of the Clyde, and 1966-1967 to 8' 116 James Mason, M. D. Nicholson, and Aileen M. Shanks

Table 42. Total landings of scallops and total effort, together with catch per unit effort as calculated from samples of selected vessels for the three areas west side of the Clyde, east side of the Clyde and west of Kintyre

Landings Effort c.p.u.e. Landings Effort c.p.u.e. Year Year (tonnes) (100 hours) (kg/10 m h) (tonnes) (100 hours) (kg/10 m h) a) West side of the Clyde 1970-1971 81 18 105 17 85 1966-1967 88 25 91 1971-1972 31 1 4 1967-1968 93 20 88 1972-1973 - 8 1 127 1968-1969 123 48 76 1973-1974 7 145 1969-1970 142 53 31 1974-1975 39 1970-1971 270 74 57 1971-1972 133 51 106 c) West of Kintyre 1972-1973 68 24 107 1966-1967 451 68 208 1973-1974 106 22 74 1967-1968 884 125 214 1974-1975 88 17 - 1968-1969 1 918 258 203 1969-1970 1 549 227 161 b) East side of the Clyde 1970-1971 556 101 134 1966-1967 33 8 47 1971-1972 728 169 98 1967-1968 34 10 47 1972-1973 317 73 116 1968-1969 14 5 41 1973-1974 578 108 134 1969-1970 38 7 - 1974-1975 1 153 207 127

Table 43. Estimated numbers of scallops landed by age in the three areas west side of the Clyde, east side of the Clyde and west of Kintyre

Year 1963- 1964- 1965- 1966- 1967- 1968- 1969- 1970- 1971- 1972- 1973- 1974- Age 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 a) West side of the Clyde (00s) 2+ 487 311 161 77 52 48 119 - 63 94 781 3 + 2 374 992 591 277 52 359 423 1 039 184 625 76 4 + 3 775 905 1 064 865 356 140 207 1 078 653 397 116 5+ 2 755 701 616 1 147 1 085 182 176 860 621 528 388 N o data 6+ 2 405 429 751 598 1 264 743 676 2 002 507 375 242 7+ 1 096 375 545 440 646 1 672 1 374 3 157 1 426 287 348 8+ 1 065 282 287 285 373 976 1 559 2 143 1 400 388 1 119 9+ 426 14 134 168 154 576 991 975 729 202 1 391 > 9 + 837 409 582 321 427 1 143 1 251 1 591 760 362 575

b) East side of the Clyde (00s) 2 + 37 20 37 14 5 2 14 - 3 - 3+ 169 254 280 27 67 13 25 47 66 - 4+ 756 754 865 79 79 22 110 323 91 4 5+ 711 1 440 1 251 135 86 47 90 600 69 11 No data No data 6+ 539 1 052 1 181 195 174 42 145 978 187 7 7+ 551 573 968 266 231 68 210 592 432 5 8+ 618 722 925 240 298 75 308 611 315 2 9+ 283 297 586 155 308 110 586 267 171 1 > 94- 292 388 828 438 387 280 320 439 132 6

c) West of Kintyre (00s) 2 + _ 252 639 664 239 208 468 1 459 3 631 3 + 687 1 052 1 918 3 908 1 088 1 109 483 2 505 11 608 4+ 902 3 240 2 923 6 121 2 336 2 808 814 3 028 6 877 5+ No data No data N o data 2 061 3 198 5 024 7 006 2 150 4 402 1 870 2 477 4 181 6+ 1 009 3 829 10 688 7 964 2 044 3 917 2 353 4 514 3 961 7+ 1 653 3 703 9 958 10 914 1 805 4 645 2 262 4 046 5 336 8+ 1 267 3 914 9 501 8 407 1 752 4 264 1 840 2 698 4 456 9+ 1 374 3 661 7 948 6 121 2 283 2 531 1 026 1 349 3 246 > 9 + 12 517 19 231 42 753 22 639 12 795 10 780 3 982 5 450 11 608 A comparison of exploited populations of the scallop, Pecten maximus (L.) 117

1974-1975 for west of Kintyre. These data are given Table 45. Mean muscle weights at age for scallops in Table 43a, b,c. Estimates of fishing mortality at from the west side of the Clyde, east side of the age for each year were obtained from virtual popu­ Clyde and west of Kintyre lation analyses (Gulland, 1965). The estimates for 1965-1966 to 1969-1970 for the east and west sides f------Mean muscle weights (g)------^ of the Clyde and for 1968-1969 to 1971-1972 for Age West side of Clyde East side of Clyde West of Kintyre west of Kintyre were averaged and were found to 2 + 6-2 9-8 4-5 follow a sigmoid curve with increasing age. The 3+ 13-7 17-2 15-0 estimates were smoothed to follow the curve more 4+ 18-5 22-5 23-0 closely, and are given in Table 44. The increase of 5+ 21-1 26-7 29-0 fishing mortality with age will be a function of 6+ 22-7 30-5 32-5 7+ 24-9 33-2 34-5 several factors, such as partial recruitment and gear 8+ 26-2 36-8 37-0 selectivity and efficiency. 9+ 26-9 38-7 39-5 A value of the natural mortality coefficient M, > 9 + 27-7 40-5 41-0 assumed constant with age, is required to carry out the virtual population analyses. Several values were ductor muscle weights, which are given in Table 45. chosen, and it was found that the results were rela­ YPR and BPR are therefore in terms of muscle tively insensitive to changes in M over the range weight. 0-1 to 0-2. M was therefore fixed at 0-15. The terms in the summations for YPR and BPR From the fishing mortalities given in Table 44 it corresponding to the 1 + scallops were in fact omitted. is possible to calculate two quantities, the yield per The weight of the adductor muscle for these scallops recruit (YPR), defined as is insufficient for them to provide a commercial yield, and BPR can therefore be interpreted as reflecting YPR = 2 F i exp 2 <&) [1 - exp (-& )] W tfZt the salable yield per recruit. Also, since scallops are beginning to mature at 2+ (Mason, 1958) BPR can and biomass per recruit (BPR), defined as be taken to give an indication of spawning biomass per recruit. In addition to calculating YPR and BPR for the BPR = 2 exp ( - 2 Zi) m arrays of fishing mortality coefficients given in Table i \ i = i I 44, they may also be calculated for fishing mortality where the summation over i refers to summation over coefficients derived from these arrays in various ways. the lifespan of the scallop, which was taken to be 19 Two ways in particular are of interest: years, Zi = Fi + M is the total mortality rate in its 1-th year, F{ is the fishing mortality rate in its z’-th a) the complete array of fishing mortalities at age can year, and Wi is the mean weight in its i-th year. be multiplied by a constant K, which if less than The mean weights were taken to be the mean ad- unity will correspond to a decrease in fishing effort and if greater than unity to an increase in fishing effort, and Table 44. Fishing mortalities at age estimated by the b) the array can be shifted relative to the ages for method of virtual population analysis and averaged which it was calculated, which corresponds to over the years 1965-1966 to 1969-1970 for the west changing the age at first capture. and east sides of the Clyde and 1968-1969 to 1971-1972 for west of Kintyre YPR and BPR were calculated for a range of fishing mortality arrays corresponding to f ------Average fishing mortality at age ------^ Age West side of Clyde East side of Clyde West of Kintyre a) multiplying the arrays in Table 44 by values of K varying from 0 to 4, and 1+ 0 0 0 2+ 0-022 0-012 0-030 b) shifting the age location of the arrays by ± 1, 2, 3+ 0-035 0-045 0-080 3, 4 and 5 years, with the proviso that fishing 4+ 0-052 0-085 0-132 mortality for 1 + scallops was always zero. Although 5+ 0-078 0-125 0-188 these shifts do not correspond exactly to the usual 6+ 0-118 0-175 0-255 7+ 0-182 0-245 0-295 concept of changes in age at first capture since 8+ 0-228 0-360 0-300 by definition scallops at age less than 2+ do not 9 + 0-240 0-440 0-300 contribute to yield or biomass per recruit, it is > 9 f 0-240 0-450 0-300 convenient here to refer to these shifts as such. James Mason, M. D. Nicholson, and Aileen M. Shanks

3 a o

O o O)4) O O c 0) c01 c o u-C VJZ

0-5

4) 3 C. CL o O

o O 0)

100 0) 01 m 110 c eo> JCo o 120 100 U U 130 UO

0-5 0-5 (b) K Figure 69. Isopleths of (a) yield per recruit and (b) biomass Figure 70. Isopleths of (a) yield per recruit and (b) biomass per per recruit. West side of the Clyde. (Units - grammes of recruit. East side of the Clyde. (Units - grammes of adductor adductor muscle). muscle).

Negative shifts correspond to an earlier age at first of the Clyde, east of the Clyde, and west of Kintyre. capture, and obviously if taken far enough, wiJl The interactions between the level of fishing effort and have the effect of fishing all scallops of 2+ and age at first capture, however, can more easily be seen older at the highest rate. by looking at the YPR and BPR isopleth diagrams. The essential feature of the YPR diagrams for all Yield per recruit and biomass per recruit isopleth three areas is a ridge of maximum yield which is diagrams were then drawn and are shown in Figures attained for values of K of approximately 1-75 and 69, 70, and 71. The yield contours relate values of above, and for changes in age at first capture of K and shifts in age at first capture having equal levels between —3 and +2 for west of the Clyde and for east of YPR. Similarly, the biomass contours demonstrate of the Clyde, and between 0 and +2 for west of this interaction between the level of fishing effort and Kintyre. From the asymmetric orientation of these age at first capture in terms of the biomass per recruit. ridges, it can be seen that there is some interaction Current levels of YPR and BPR occur with K = 1 between K and change in age at first capture, and and change in age at first capture equal to zero. that to increase YPR, modifications to K should be Yield per recruit curves showing the variation in accompanied by modifications to the age at first cap­ yield with changes in the level of fishing effort can be ture. For the east and west sides of the Clyde increases obtained by taking a section through the YPR surface in the level of fishing effort would need to be accom­ with change in age at first capture held constant. panied by an earlier age at first capture, whereas for Figure 72a, b and c show yield per recruit curves ob­ west of Kintyre there would need to be a corresponding tained for no change in age at first capture for west increase in the age at first capture. A comparison of exploited populations of the scallop, Pecten maximus (L.) 119

Y P R(grammes)

10-

Ic ■ft

0 5

12- Y P R (grammes) UI 10-

2-1

0-5 K

YPR (grammes) 1 2 -

1 0 -

Vi 6 - ■B

.£

I 120 0-5 K Figure 72. Yield per recruit curves for no change of age at first capture for (a) west side of the Clyde, (b) east side of the Clyde b and (c) west of Kintyre. Figure 71. Isopleths of (a) yield per recruit and (b) biomass per recruit. West of Kintyre. (Units - grammes of adductor muscle). isopleth diagrams we can see that the effect of moving From the yield isopleth diagrams it can be seen that from A to A' and B to B' for the west and east sides to achieve the ridge of high yield without moving too of the Clyde would be to decrease considerably the far from the current state of exploitation, an increase BPR, whereas for west of Kintyre moving from C to C' in the level of fishing effort of 100% (i.e. K = 2) would result in only a slight decrease. The biomasses and a decrease of 2 years in the age at first capture in terms of muscle weight per recruit would be are required for the west side of the Clyde, a 75% increase in fishing effort and a decrease of 1 year in Present Projected age at first capture for the east side of the Clyde, West Clyde 72 42 and an increase of 75% in fishing effort and an East Clyde 85 57 increase of 1 year in the age at first capture for west West of Kintyre 76 71 of Kintyre. These changes are signified by the lines A to A', B to B' and C to C' respectively. This would Since BPR refers to the spawning biomass, these give the following yields of grammes of muscle per extensive reductions for the east and west sides of the recruit. Clyde might affect the level of recruitment, and so Present Possible lead to a reduction in the overall yields from these West Clyde 7-3 9-2 fisheries. This is not so for west of Kintyre. East Clyde 11-3 12-3 If we examine the average landings in these areas West of Kintyre 11*3 12-6 for the first five years of the present decade (for which the level of fishing effort is comparable to that for However, we must also consider the effect these which the yield and biomass per recruit figures were changes would have on the biomass. From the BPR calculated), then assuming a muscle to total weight 120 James Mason, M. D. Nicholson, and Aileen M. Shanks

ratio of 15% the total number of recruits and biomass ACKNOWLEDGEMENTS may be estimated. Total annual recruitment and bio­ We gratefully acknowledge the advice given by our mass in terms of total weight were as follows: colleagues Mr J. A. Pope and Mr R. Jones during the preparation of this paper. Recruits Biomass (tonnes) West Clyde 2-73x10« 1 311 REFERENCES East Clyde 0-42 xlO6 240 West of Kintyre 8-85 xlO6 4 484 Gulland, J. A. 1965. Estimation, of mortality rates. [Annex to] Report of the Arctic Fisheries Working Group (meeting in Hamburg, Jan. 1965). ICES CM 1965, Gadoid Fish Cttee, As can be seen the biomass for west of Kintyre, the Doc. (3) (mimeo). newest of the three fisheries, is by far the largest. Mason, J. 1957. The age and growth of the scallop, Pecten maximus That for the east of the Clyde is by far the (L.) in Manx waters. J. mar. biol. Ass. U.K., 36: 473-92. Mason, J. 1958. The breeding of the scallop, Pecten maximus (L.) smallest. in Manx waters. J. mar. biol. Ass. U.K., 37: 653-71. Mason, J. 1972. The Scottish fishery for scallops and queens. Scottish Fisheries Information Pamphlet. Aberdeen, Marine CONCLUSIONS Laboratory, 12 pp. The results from the YPR analyses suggest that for Mason, J. and Drinkwater, J. 1969. Scallops (Pecten maximus (L.)) all three areas it would be possible to increase yield in the Firth of Clyde. ICES CM 1969/K: 53, 3 pp. (mimeo). Mason, J. and Drinkwater, J. 1974. The stocks of scallops, Pecten per recruit through an increase in the level of fishing maximus (L.), in the Clyde sea area and west of Kintyre. Annls effort and an appropriate change in age at first cap­ biol., Copenh., 29: 184-6. ture. However, an examination of the BPR isopleth Mason, J. and Drinkwater, J. 1975. The stocks of scallops, Pecten diagrams suggested that for the east and west sides maximus (L.), in the Clyde sea area and west of Kintyre in 1972-73. Annls biol., Copenh., 30: 213-4. of the Clyde these changes would result in a decline Mason, J. and Drinkwater, J. 1976. The stocks of scallops, Pecten in biomass per recruit, a drop in the level of recruit­ maximus (L.), in the Clyde sea area and west of Kintyre in ment, and hence a lower overall yield. 1973-74. Annls biol., Copenh., 31: 183-4. The evidence suggests a present low level of biomass in the Clyde, especially on the east side. The water RÉSUMÉ OF DISCUSSION ON THIS AND THE currents in the Clyde sea area and west of Kintyre PRECEDING PAPER during the late spring and summer, when storms are Dr Mason stated that samples were not available least likely, are weak and cyclic (H. D. Dooley, per­ in the early stages of exploitation so that it had not sonal communication). The larvae resulting from been possible to arrive at a value for M using the spawnings in these areas can therefore generally be catch curve. He expressed the opinion that at low expected to augment the stocks broadly in their area levels of stock size recruitment was probably related of origin. Assuming recruitment to be related to stock to stock size. Dr Caddy reported that for Placopecten size, which is most likely at a low level of biomass, intraspecific space competition limited stock recruit­ then allowing BPR to increase should lead to an ment, there being a maximum density of 6/m2. Mr increase in recruitment and, it is hoped, in overall Posgay stated that in tanks scallops sorted themselves yield also. Various combinations of change in age at to give a uniform spacing. Mr Naidu questioned the first capture and in the level of fishing effort would validity of relating recruitment to stock size where result in an increase in BPR, but in view of the two separate spawning stocks are involved. Dr Mason present low level of fishing effort, particularly on the agreed that any management measures would have east side of the Clyde, it would be preferable to achieve to cover both stocks. Mr Jones indicated the pitfalls this increase solely by raising the age at first capture in combining the concepts of yield and yield per rather than by changing the fishing effort. This could recruit. In response to a question by Mr Naidu be achieved either by the introduction of a minimum Dr Caddy expressed the opinion that pulse fishing legal landing size or by introducing dredge mesh and would not be a practical means of management for tooth spacing regulations. the Georges Bank fishery.