ICES mar. Sei. Symp., 192: 99-107. 1991

A large-scale experiment to enhance a stock of lobster (Homarus gammarus L.) on the English east coast

R. C. A. Bannister and A. E. Howard

Bannister, R. C. A ., and Howard, A. E. 1991. A large-scale experiment to enhance a stock of lobster (Homarus gammarus L.) on the English east coast. - ICES mar. Sei. Symp., 192: 99-107.

Scientists from the Ministry of Agriculture, Fisheries and Food arc conducting a long­term experiment on the English east coast to test the potential for enhancing the stocks of the lobster (Homarus gammarus L.) using hatchery-reared juveniles. During the release phase from 1983 to 1988 over 50 000 microtagged lobsters were released onto lobster habitat at the east coast site, which is also fished commercially. Moni­toring of commercial catches has now commenced, with positive results. This paper summarizes the rearing, microtagging, release, and monitoring programmes, and describes the results obtained so far. It also discusses future monitoring objectives and examines the prospects for enhancement in the context of the lobster stock- recruitment relationship.

Les scientifiques du Ministry of Agriculture, Fisheries and Food ont entrepris une expérience à long terme sur la côte est de l'Angleterre pour tester la possibilité d'augmenter les stocks de homards par immersion de juvéniles produit en écloserie. De 1983 à 1988 près de 50 000 homards juvéniles ont été lâchés sur un site où s’exerce une pêche commerciale. Chaque juvénile a été marqué par une «microtag». La phase d’examen des captures commerciales est désormais entreprise au débarquement pour retrouver les homards marqués, et quelques résultats positifs ont été obtenus. Ce papier décrit le programme d’élevage, de marquage, de libération et d’échantillonnage des recaptures, ainsi que les résultats préliminaires. En outre, il décrit d'autres objectifs pour l'échantillonnage à l’avenir, et commence un débat prospectif con­cernant l’efficacité de l’augmentation par rapport à la relation entre le stock et le recruitement du homard.

R. C. A . Bannister and A . E. Howard: Ministry o f Agriculture, Fisheries and Food, Directorate o f Fisheries Research, Fisheries Laboratory, Lowestoft, Suffolk NR33 0HT, UK.

Introduction

Scientists of the Ministry of Agriculture, Fisheries and Food (M AFF) have completed the release phase of a long-term experiment to test the potential to increase stocks of the lobster, Homarus gammarus L., in the United Kingdom. From 1983 to 1988 over 50 000 microtagged hatchery-reared juveniles were released onto lobster habitat on the English east coast between Bridlington (54°5'N 0°11'W) and the Humber (Fig. 1), to mingle with naturally-settled lobsters supporting a substantial commercial trap fishery. It is hoped that in this favourable habitat enough hatchery-reared stock will survive to contribute significantly to commercial catches and so assist in evaluating the technical and

economic feasibility of this type of enhancement project. Preliminary monitoring trials began at Brid­lington in 1987, and in 1988 they yielded 26 microtagged recaptures originating from batches released in 1983 and 1 984 -an important initial result. (Since the drafting of this paper a further 110 microtagged lobsters were recaptured in summer 1989; these are described briefly in the appendix.) This paper summarizes the M AFF release and recapture programmes, discusses aspects of the future monitoring strategy, and raises the question whether hatchery-reared stock will enhance, or merely substitute for, local recruitment.

This work is part of a broader programme incor­porating similar lobster releases by two other groups, one operating in Scotland and the other in Wales, as noted below. The programme is coordinated by the

99

MAFF Coordinator of Fisheries R&D Working Group on Lobster Stock Enhancement.

Material and methods

The key features of this experiment are the use of identifiable animals directly placed onto favourable sea­bed habitat and subsequently identified by screening commercial catches using a quayside tag detector. The details are as follows.

1. R earing and microtagging

Wild ovigerous females were held to produce larvae in the hatchery at the M A FF Fisheries Laboratory, Conwy, North Wales. Larvae were reared at 18-21°C to reach juvenile stage X to XII at approximately 11 to 15 mm carapace length (CL) after three months (Beard et al. , 1985). Such juveniles are large enough to tag with a binary coded wire microtag (1 x 0.25 mm) (Jefferts et al., 1963), implanted by modifying proprietary salmon tagging methods (Wickins et al.., 1986). Tags were inserted at the base of the 5th pereiopod (which will not be consumed by the public) and except in 1983 when tag loss was 32% tag retention over succeeding moults was above 90% in laboratory trials (Wickins et al., 1986). Spring and autumn releases in each year were

distinguished by inserting the microtag into the left or right leg respectively.

2. The release site and strategy

Commercial lobster stocks occur mainly in rocky habitat, whilst juvenile and adult lobsters show adaptive behaviour aimed at finding and inhabiting suitable sub­strate to obtain protection from strong water flows and from predation (Cobb, 1971; Botero and Atema, 1982; Howard and Bennett, 1979; Howard, 1980; Howard and Nunny, 1983; Howard, 1982, 1983). The release site and methodology were chosen to take advantage of these observations.

South of Bridlington commercial lobster catches come from a large area of boulder and cobble reefs distributed out to at least 7 miles offshore along a 15-mile length of coast from Skipsea to Withernsea (Fig. 1). The release zone comprises the inshore part of this area out to 4 miles and depths of 10-15 m. From 1983 to 1988 batches of up to 7000 juvenile lobsters were released twice a year - in May-July and September-October. Lobsters in batches of several hundred were scattered by divers directly onto previously marked reefs whose position was recorded by Decca Navigator on the release boat (Fig. 1). A t summer temperatures (14-16°C) lob­sters released by diver close to cobble substrate engage

J 1---------1---------1---------1--------1---------1_____I______I_____I

Skipsea

5 8 ’ -

Atwick5 6 ' -

Hornsea5 4 ' -

Mappleton5 2 ' -

Aldborough50 ' -

Grimston4 8 ' -

Spring Autumn4 6 ' -

1983 + o°o SVWithernsea

44 ' -8788

53'42'

Figure 1. Lobster stock enhancement. Location of the lobster release area and distribution of the main release reefs, Bridlington, East Yorkshire.

Aberystwyth

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Table 1. Lobster stock enhancement in Britain. Releases of microtagged hatchery-reared juveniles (1983-1988).

MAFF“ Bridlington Bay

Spring Autumn

NW & NWSFCb Aberystwyth

Spring AutumnSFIA1

Ardtoe Scapa Flow

Numbers corrected for tag lossdNot corrected for

tag loss6

1983 268 2122 _ _ _

1984 2 418 6198 - 1250 480 4 7051985 6 019 1 960 2100 1660 1335 4 0001986 5 480 6 082 2 438 1863 540 2 4801987 6 016 6 613 1036 2 180 582 3 8001988 - 5 952 2100 4 606 2 657 2 374

20 201 28 927 7 674 11559 5 594 17 359Grand total 49128 19 233 22 953

a Ministry of Agriculture, Fisheries and Food. b North Western and North Wales Sea Fisheries Committee. c Sea Fish Industry Authority.d Total numbers minus the estimated tag loss (0-6%) measured by Wickins et al. (1986). ' Different tagging operators.

in shelter-seeking behaviour and become established in crevices and burrows (Howard, 1983).

The total number of lobsters released is shown in Table 1, and compared with the number released by the other two experimental groups. M AFF data have been corrected for the small rate of tag loss (0-6% over several trials) estimated by Wickins et al. (1986), but the other group releases are not corrected because the operators were different.

3. M onitoring

The monitoring phase involves the capture of lobsters of various sizes in order to test for the survival ofhatchery-reared stock.

Options depend on the ecology of lobsters. Juveniles up to 40 mm seem mainly to shelter and feed in their burrows (Cooper and Uzmann, 1980). At this size lob­sters can only be sampled by searching individual reefs by diver (Walker, 1986), but for economic reasons MAFF has not used this approach systematically. Larger juveniles, however, emerge to forage and above 50 mm can be caught in substantial numbers in commercial prawn and lobster traps. Initial MAFF monitoring efforts, therefore, concentrated on commercial trap sampling for undersized (pre-recruit) lobsters in the peak fishing season from July to September. First, selec­ted fishermen working in the study area were permitted to land undersized lobsters at the quay as a derogation from the minimum landing size regulation. The precise capture position of these lobsters was not known. Second, undersized lobsters were caught at known posi­tions during seagoing visits by scientific staff. Under­

sized lobsters brought to the quayside by the two methods were measured, sexed, and passed through a tag detector manufactured by North West Marine Technology of USA. (This name is for information only. It does not signify any form of commercial endorsement of the product by M AFF.) Lobsters giving a positive response were set aside, labelled, and frozen for return to Lowestoft. Tag presence was later confirmed by X- ray, dissection, and decoding of the release information. Remaining animals were held in commercial holding tanks overnight and returned to sea the next day. Third, commercial sized lobsters were tested directly at the landing place on a few occasions. In all these cases the landing place was at Bridlington.

4. Studies in Scotland and Wales

Stock enhancement trials are also being undertaken by the Sea Fish Industry Authority (SFIA), and the North Western and North Wales Sea Fisheries Committee (NWNW SFC). These groups have taken up MAFF methodology, with variations. SFIA lobsters, reared at the Marine Farming Unit, Ardtoe, northwest Scotland, have been released by diver both at Ardtoe (56°45'N 5°50'W) and at a dozen or so sites in Scapa Flow, Orkney (58°53'N 3°10'W approx.). The NWNW SFC lobsters, reared at the University College of North Wales, Menai Bridge, have been released at two sites off Aberystwyth (52°25'N 4°5'W approx.). In the Aber­ystwyth release, lobsters were carried to the seabed in a flow of sea water running down a flexible pipe, which disperses lobsters more widely but less precisely (A non., 1984). The total numbers of lobsters released by these

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Table 2. Size and recapture details of MAFF recaptures of microtagged lobsters in Bridlington Bay (July to September 1988) arranged by release date.

Release date: July 1983 October 1983 July 1984 September 1984

Female Male Female Male Female Male Female Male

69 (4) 6 5(2 ) 75 (2) 52 (1, 3, 5) 65(4) 7 1 ( 1 ,3 ) 65 (1, 3)

71 (2) 68 (2) 78 (2) 69 (1) 69(1) 72 (I , 3) 76 (1, 3)

81 (2) 69(1) 82 (4) 78 (4) 72 (2, 3)69(2) 72 (4)69(2) 74 (1, 3)69 (2)69 (2)75 (4)

Recapture codes: 1. Spot recapture position known. 2. Approximate recapture position known. 3. Release and recapture positions inconsistent. 4. Recapture position not known. 5. Diver caught.

groups are included in Table 1. Both groups have moni­tored their sites using fleets of prawn and lobster traps fished on the release reefs. In addition, SFIA made intensive diver searches at Ardtoe and Scapa in 1985 (Walker, 1986).

Results

No microtagged lobsters were detected during trial

screening by M AFF of 713 undersized lobsters at Brid­lington in 1987. In 1988, however, 25 undersized microtagged lobsters were detected out of 3200 under­sized and 308 commercial-sized lobsters tested at Brid­lington, and one small microtagged lobster was also caught by diver. In Table 2 these M AFF recaptures are summarized by release date, sex, and size at recapture, and with an indication of the reliability of the recapture position. Table 3 lists the recaptures by capture date, and shows the areas of release and recapture.

Table 3. Lobster stock enhancement in Britain. List of MAFF recaptures in 1988 in order of capture, with release and recapture positions.

Capture Size Date ofdate (mm) Sex release Release area Recapture area Comment

28.7.88 75 F 10/83 Hornsea-Mappleton Uncertain31.7.88 65 M 7/84 Hornsea -

1.8.88 65 F 10/83 Hornsea-Mappleton Hornsea (1 m)5.8.88 69 F 10/83 Hornsea-Mappleton Hornsea-Mappleton

11.8.88 69 F 10/83 Hornsea-Mappleton Hornsea-Mappleton17.8.88 69 F 7/83 Hornsea-Mappleton -17.8.88 72 F 9/84 Hornsea (5 m) or Aldborough Hornsea22.8.88 68 F 10/83 Hornsea-Mappleton Mappleton (1 m)22.8.88 69 F 10/83 Hornsea-Mappleton Mappleton ( 1 m) y Same fleet of traps22.8.88 69 F 10/83 Hornsea-Mappleton Mappleton (1 m)22.8.88 71 F 7/83 Hornsea-Mappleton Mappleton (1 m)23.8.88 72 F 9/84 Hornsea (5 m) or Aldborough -23.8.88 78 F 7/84 Hornsea -

25.8.88 69 F 10/83 Hornsea-Mappleton Mappleton (1 m) '25.8.88 78 M 10/83 Hornsea-Mappleton Mappleton (1 m) ► Same fleet of traps25.8.88 75 M 10/83 Hornsea-Mappleton Mappleton (1 m) .25.8.88 82 M 10/83 Hornsea-Mappleton - As above?

5.9.88 69 F 7/84 Hornsea Hornsea (3 m)5.9.88 74 F 9/84 Hornsea (5 m) or Aldborough Hornsea (3 m) 1

Same fleet of traps5.9.88 69 M 7/84 Hornsea Hornsea (3 m) J7.9.88 72 F 9/84 Hornsea (5 m) or Aldborough Hornsea (3 m)7.9.88 71 F 9/84 Hornsea (5 m) or Aldborough Hornsea (3 m) 'i7.9.88 76 M 9/84 Hornsea (5 m) or Aldborough Hornsea (3 m) I Same fleet of traps7.9.88 65 M 9/84 Hornsea (5 m) or Aldborough Hornsea (3 m) J

11.9.88 52 F 7/84 Hornsea Grimston Diver caught12.9.88 81 F 7/83 Hornsea-Mappleton Mappleton (1 m)

m = nautical miles.

102

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Tag decoding showed that all the 1988 recaptures were released in 1983 and 1984. The location of the microtag, whether in the left or right pereipod, further classified the lobster as a spring or autumn release, although in five cases the microtag was found fairly close to the mid-line of the animal.

Spot recapture positions (from Decca Navigator read­ings) were known for 9 animals, and visual bearings gave an approximate position for a further 11 animals (bearings taken from a small boat off a low coastline being somewhat inaccurate). Altogether 19 recaptures were from the Hornsea-Mappleton area (Fig. 1). Despite substantial trapping further north and south only one lobster, caught by diver, was taken elsewhere, at Grimston. As Table 3 shows, the same fleet of traps, hauled three days apart, caught groups of four and three recaptures respectively, whilst off Hornsea another fleet, hauled two days apart, caught groups of two and three recaptures respectively. Overall, therefore, there was a strong clustering of recaptures in the H ornsea- Mappleton area and on individual grounds. Establishing the link between recapture and release area is not always straightforward because lobster microtags, though batch coded, are not individually identifiable. Of the 19 Hornsea-Mappleton recaptures, however, 13 belonging to the 1983 and July 1984 releases are wholly consistent with the release positions, which were all in the Hornsea-Mappleton area. The remaining recaptures, which belong to the September 1984 release, are more problematical. This release included 1240 lobsters lib­erated off Hornsea, but several miles further offshore than the recapture position. The remainder of this batch, however, a further 5000 lobsters, were liberated several miles to the south off Aldborough and we cannot exclude this objectively as the source of the Hornsea recaptures of the September 1984 release. The diver- caught lobster recaptured off Grimston was several miles south of the July 1984 release position at Hornsea, the only recapture out of 26 with definite evidence of movement.

From these results the conclusion is that a majority of recapture positions were close to the position of release; that one recapture position was more than two miles south of the release position; and that although the remaining six recapture positions could be in the general vicinity of their release position it cannot be excluded that their release position was several miles further south.

Recapture data for the other groups are not yet fully available but a summary is included together with the MAFF data in Table 4. Recaptures from Scapa Flow (SFIA) include captures by diver and traps at all sites, all of them close to known release positions. Recaptures off Aberystwyth (NWNW SFC) were made by a limited trapping trial and were again close to the release position. Most recaptures by all three groups were in the size range 40 to 70 mm but smaller animals were

103

obtained from the SFIA diver survey in 1985 and two larger animals close to commerical size were caught at Bridlington and Scapa Flow. We are indebted to Mr C. Burton (SFIA) and Mr W. Cook (NWNW SFC) for assistance with the non-MAFF data.

These results, though preliminary, are encouraging. They show that:

(1) some hatchery-reared lobsters can survive in the wild for at least five years;

(2) hatchery-reared lobsters will enter and stay in com­mercial traps;

(3) microtags can be detected, dissected and decoded after five years at liberty;

(4) recaptured lobsters have shown substantial growth which is broadly comparable across sites, two lob­sters reaching very close to commercial size;

(5) a majority of the known recapture positions are clustered close to the release positions, but for six recapture positions this cannot be verified.

Discussion

1. Assessing abundance

Initially, monitoring aimed to assess sampling logistics and to obtain evidence that microtagged lobsters sur­vived and were detectable in catch samples. In this it has been successful, and together with the additional information obtained on growth and recapture positions the results strongly suggest it is worthwhile to continue with the monitoring programme until recaptures have been obtained from each release cohort, say by 1993.

Possible next steps in the assessment of results are to attempt to estimate the commercial catch from each release cohort, and to estimate if possible the actual stock abundance and hence the survival rate of each cohort.

Estimating how many microtagged lobsters are caught each year by the commercial fleet tests whether their value will, on average, exceed production and release costs (Howard, 1982) irrespective of whether this rep­resents a net addition to the stock or the catch. This is the simplest assessment and monitoring option. It requires that we screen every lobster landed com­mercially, or a representative sample whose detection rate can be raised to total landings. The subsampling approach assumes that vessels sampled will be from the same time-space strata as the rest of the fleet. The spot position of recapture is not important.

More difficult is estimating the total number of each hatchery-reared cohort surviving to a particular size in order to calculate and compare pre-recruit survival rates across cohorts and release seasons. One approach to this is to convert the estimated commercial catch of microtag cohorts to a stock estimate using the catch equation and an estimate of fishing mortality derived

from size composition analysis. This would depend for accuracy on the precision of the catch estimate, and an absence of bias in the estimation of fishing mortality due to such non-fishing factors as can affect size composi­tion analysis (Addison, 1986). Alternatively one may attempt to estimate total lobster abundance by size class and subarea using a separate mark-recapture experi­ment, and thence to partition this into hatchery reared and microtagged portions using the ratio of microtagged and untagged lobsters in subarea catch samples. A t this stage it is not clear how accurate such an approach could be. Target fishing on individual reefs would be needed. This may not be very precise, and results will be subject to spatial and sampling variability resulting from both the patchy distribution of lobster habitat (with associ­ated abundance and size composition differences), and the interaction between sample spacing and the foraging behaviour of individuals. Results could also be affected by seasonal changes in availability (due to temperature and moulting) coupled with seasonal depletion due to exploitation. Trials are currently in progress to inves­tigate the logistics of conducting an experiment of this type.

2. Stock and recruitm ent considerations

An important future consideration is whether the sur­vival of microtagged lobsters will represent enhance­ment of the stock, or whether it will represent survival which has occurred at the expense of a higher mortality of naturally settled lobsters in the same area, i.e. a substitution effect. The present experiment is large scale, and the local habitat is settled regularly by natural recruitment. It is conceivable that in this case the hatch­ery reared cohorts will be more strongly engaged in competition for space and food resources than if lobsters had been added to new habitat such as an artificial reef, or to habitat which, though suitable for lobsters, is either on the margins of existing recruitment, or else carries a very low natural stock because of excessive depletion by fishing. In this experimental area juveniles settle regularly and survival of the hatchery reared stock will depend on the interactions between total space and food resources, the variation in occupancy due to natural variation in recruitment, and the re-cycling of lobster habitat as adult stock is reduced by fishing. We can only hazard a guess as to the likely scale of these factors. For example, since 1976 fishing effort in the experimental area has probably been fairly stable but landings have varied between 12 and 3 0 1, reaching the upper limit on two occasions, the second of which was over the last three years when natural recruitment seems to have been particularly good. In this area the average lobster is about 0.7 kg when captured, and assuming that F /Z is 0.9 based on size composition analysis (Ban­nister, 1986) the exploited stock may have varied between 20 000 and 50 000 commercial-sized animals

104

(probably comprising two or three main year classes) as year-class strength has varied. At stage IV the natural stock will be larger than this but we cannot say by how much. Even so, this suggests that annual releases of 13 000 or so hatchery-reared lobsters were very approxi­mately of the same order as a poor year class, but probably a much smaller proportion of a good year class. The question of whether this will give rise to significant competition obviously depends on whether or how density-dependent regulatory factors control the balance between the number of adults, population fecundity, and the “normal” level of settlement, as well as the factors which allow the observed variations in year-class strength to occur.

There is no direct knowledge of pre-recruit ecology in this experimental area, but the scope for competition will be reflected in the general shape of the lobster stock- recruit relation, and the precision of any regulatory mechanism will be indicated by the variance of this curve. Fishermen tend to assume that if an adult popu­lation is depleted, recruitment is necessarily also affec­ted, thereby making space available for enhancement. In fact, recruitment overfishing is not clearly demon­strated for lobster stocks, which do not appear to col­lapse easily (Fogarty, 1980; Bannister, 1986; Bannister and Addison, 1986). The implication is that the relation between stock and recruitment is compensatory. Infor­mation on this is lacking for stocks of Homarus gammarus, but for Homarus americanus in the Nor­thumberland Strait region of the Gulf of St Lawrence, Canada, information on the abundance of larvae and of adults has been analysed by Fogarty and Idoine (1986), who show that the resulting stock-recruitment curve is asymptotic. Further, they showed that compensation did not appear to occur in the planktonic phase, but must occur at or after the stage IV settlement stage. A density-dependent process or effect must occur as stage IV juveniles attempt to find and settle into the benthic phase, or at some life history phase between settlement and adulthood. In Figure 2 we propose a hypothetical cohort survival schedule of mean log numbers against

. P l a n k t o n i c S tag e

M in im umla nd in g- S e t t l e m e n tL o g a r i t h m

n u m b e rs

O pen fo r a g in gC rev ic e S tagei F ishery

90 10060 7020 30 40

Size ( c a r a p a c e l e n g th in mm)

Figure 2. Lobster stock enhancement. Schematic repre­sentation of mean cohort survival against size.

Figure 3. Lobster stock enhancement. Schematic repre­sentation of survival for the upper and lower limits of cohort abundance assuming (A) density-dependent settlement, and (B) density-dependent post-settlement mortality.

size, illustrating mortality features which may occur during the life history. Sequentially, this represents a rapid mortality as planktonic larvae; a major mortality at settlement as some larvae fail to find suitable substrate; a reduced mortality in a sheltered crevice phase where lobsters spend most of their time protected in burrows (Cooper and Uzmann, 1980); a slightly increased mortality when juveniles emerge to forage; and a final further increase in mortality due to exploi­tation. Figure 3 illustrates the concept of how density dependence at settlement alone, or both at settlement and in the crevice stage, might compensate for dif­ferences in the initial abundance of the planktonic phase. If density dependence were to occur primarily during the act of settlement, then the present experi­mental arrangement incorporating direct placement of juveniles on the seabed in the correct habitat, poten­tially by-passes this effect. If density dependence occurs through competition for space and food at entry into the crevice phase, or as lobsters grow through this phase, then hatchery-reared stock will potentially be subject to its effects just like the natural stock. The final outcome will then depend on the time-space variation in these "carrying capacity” effects, and on the interaction between hatchery cohort abundance and natural settle­ment variation. As the Fogarty and Idoine data show, positive deviations from the asymptotic stock-recruit curve do occur, and we might expect that although the upper limit to recruitment is strongly controlled, enhancement could nevertheless occur periodically in time and space in the same way as a good natural year class. Long term we may obtain some clues if we are able to measure hatchery-reared cohort abundance at recruitment with sufficient precision to describe dif­ferences between years, or systematic differences between the spring and autumn releases. Juveniles from

Pla n k to n ic S tage

Dens i ty d e p en d en t s e t t le m e n t

d e p en d en t po s t s e t t le m e n t m o r ta l i t y

Minimumla nd in gsize

Crev ice S tage

I I 1—0 10 20 30

Size (c a ra p a c e le ng th in mm)

L o g a r i t h mof

num be r

105

the spring release will have settled onto the ground before each new natural year class, whereas autumn juveniles will find the ground already partially occupied by new natural recruits.

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Appendix

Results from 1989

Since the original presentation of this paper preliminary recapture results for 1989 have become available. Sam­pling for microtagged lobsters followed a similar pattern but was intensified. Scientists collected undersized lob­sters at sea at known positions as before, but in addition a much higher proportion of the quayside landings of commercial-sized lobsters was passed through the tag detector. As a result, between mid-July and September 1989, a further 110 microtagged animals were recap­tured, a substantial increase over the 1988 results. Of these, 61 recaptures were undersized (54-84 mm) caught at sea by scientists, and 49 were legal-sized animals (85-96 mm) obtained at the landing place. Tags were X-rayed and decoded as before and were found to belong to the 1983, 1984, and 1985 release cohorts, the last-mentioned appearing for the first time. Table A .l shows the 1988 and 1989 sampling and recapture levels, and Table A .2 shows the number and size range of recaptures arranged by release cohort. The second table indicates that growth rate is varying between the cohorts, with the 1985 cohort in particular reaching commercial size much earlier than the other two.

A preliminary look at the 1989 recapture positions

Table A l . Sampling level and number of microtagged recap­tures for the M A FF lobster enhancement experiment in 1988 and 1989.

Numbertested

Microtaggedrecaptures

Undersized (caught by chartered commercial coble) 1988 3200 251989 1261 61

Legal sized (caught by chartered commercial coble) 1988 308 01989 2429 19

Legal sized (other commercial cobles sampled at landing) 1989 9631 30

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Table A2. Number and size range of recaptures of MAFF microtagged lobsters for the 1983 to 1985 release cohorts.

Size range at capture (mm) Number recaptured (age in parentheses)

Release Release :— --------------------------------------------year number 1988 1989 1988 1989

1983 2390 14 19 65-82 (5) 78-96 (6)1984 8616 12 67 52-78 (4) 54-95 (5)1985 7979 - 24 70-92 (4)

shows that recaptures from the 1984 release are now coming from both the northerly and southerly release locations of this cohort, and that recaptures of the newly recruited 1985 cohort were similarly caught at both northerly and southerly release locations. The essential ambiguity remains, however, that because individual lobsters were only coded by batch, and not by release position, analysis of dispersion and mixing has been confounded by splitting batches between different release positions. The commercial sampling results do, however, contain a large number of landings from lob­

ster grounds further offshore than 4 miles, all with zero recapture rates. This strongly suggests that seaward dispersion of lobsters from the 1983 to 1985 cohorts has not occurred.

The increased recaptures in 1989, and in particular the relatively large number of microtagged lobsters obtained from commercial vessels at the quayside, pro­vide further encouragement for the continuation of the monitoring programme, so much so that the latest infor­mation available at going to press is a total of 234 commercial-sized recaptures for the 1990 season.

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