During 1971 a team of divers studied the structure and numbers of entrances of burrows of Nephrops norvegicus in the Irish Sea off Clogherhead and counted their numbers in plots of 28sq.m area. Numbers of entrances found ranged from one to six with a peak at 3 and the densities of Nephrops norvegicus found ranged from 1 per 2.5sq.m downwards

Hillis, J P

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IRISH FISHERIES INVESTIGATIONS SERIES B (Marine) No. 12 (1974) AN ROINN TALMHAIOClITA AGUS IASCAIGH (Department of Agriculture and Fisheries) FO-ROINN IASCAIGH (Fisheries Division) DUBLIN: PUBLISHED BY THE STATIONERY OFFICE TO BE PURCHASED FROM THE GOVERNMENT PUBLICATIONS SALE OFFICE, G.P.O. ARCADE, DUBLIN. Price: 5p IRISH FISHERIES INVESTIGA TIONS SERIES B (Marine) No. 12 (1974) by J. PAUL HILLIS, A DIVING STUDY ON DUBLIN BAY PRAWNS NEPHROPS NORVEGICUS (L) AND THEm BURROWS OFF THE EAST COAST OF mELAND. L A diving study on Dublin Bay prawns Nephrops norvegicus (L) and their burrows off the east coast of Ireland by J. P. HILLIS. Abstract During 1971 a team of divers studied the structure and numbers of entrances of burrows of Nephrops norvegicus in the Irish Sea off Clogherhead and counted their numbers in plots of 28 m2 area. Numbers of entrances found ranged from one to six with a peak at 3 and the densities of Nephrops norvegicus found ranged from 1 per 2.5m2 downwards. Introductiou The suggestion that Dublin Bay prawns Nephrops norvegicus excavate and inhabit burrows in mud was postulated (though without proof) by Gauss-Garady (1912) as the reason for the swift and dramatic changes observed in numbers taken in the trawl. Burrowing behaviour has been described by Hoglund (1942), from captive observations, Priestly and Thomas (1964), by remote-control under-sea photography, Dybern and Hois-aeter (1965), by diving and captive observations, Crnkovic (1968) by captive observations and by Cbapman and Rice (1'171) and Rice and Cbapman (1971) by diving. The habit is responsible for the spectacular variations in trawl catch with light conditions described by many authors including Hillis (1'171, 1972) dealing with Irish Sea populations. Method The place of burrows in the behaviour patterns of Nephrops was investigated by programmes of diving carried out in the Irish Sea off C10gherhead in the area shown in Fig. 1, using the research vessels, Cll na Mara on 14, 15 and 16 June 1971 and Cll Feasa on 6, 7, 8, 13 and 16 September 1971. The divers were J. O. Browne, B.Sc., Fisheries Division, Department of Agriculture and Fisheries, and a team of three from Messrs. Irish Underwater Services Ltd., photography being undertaken on 13 September by C. Craig. The aims of the programme were as follows: 1. Estimation of the population density. 2. Observations on the behaviour of Nephrops and structure and distribution pattern of their burrows, includ-ing photographic observ_ations. 3. Observations on the action of the trawl and reaction of Nephrops to the trawl. Estimates of the population density were desirable in view of the variation in trawl catches, but the diffi-culty was recognised of obtaining meaningful data, due to the fact that only the periphery of the fiShing grounds is shallow enough to allow divers adequate working time on the sea bed. Observations on behaviour of N eph-raps at their burrows proved relatively simple. Considerations of human physiology limit each diver to a period of about 30-45 minutes per day at the relevant depths; each day's work was thus carried out iu the afternoon, so that observations and counts could be made in daylight with the Nephrops in their burrow. and the work could conclude if desired with a trawl haul at dusk for trawl escape reactions. All the divers and particularly the photographer commented on the turbidity of the water and softness of the bottom sediment, probably a feature inevitable in most Nephrops habitats; muoh the softest and finest bot-tom encountered was that found at the deepest dive made, at J5.0 F43.0 at 17 fm on the divers' meters, on 13 September, further inside the N ephrops inhabited area than any other dive. All positions were fixed by Decca Radin Chain navigational aid. Irish Fisheries Investigations Series B, No. 12 (1974). 3 Irish Fisheries Investigations Series B, No. 12 (1974). ••••• ..... •• •• j .... .... o Q) - ...... _---------_ ...... " ••• o •••••• " 4 E -,,' " . /0 ,*"", • ,... ---_...... . •••• • .. ' --~ E .,;' ......... __ .- ...... -- --"'- ... '1::_ ... , ~ -' t) ~ " 0 ." ~ ~ Jl 0 ~ 0 ~ 0 ." .~ 0 '" " ~ , "" -~ " 0 ~Z :; .~ ~ § " 0 " ~0 ] e 0 -" .S "" " •~ ~ ], a ~ Q Ii " 0 §~ u" f:§ ~!t "'''' on", d_ .~ II '" ~'" -C. rom ..2 r-f ~ . ",-~~f D~ "'", 0_ § II .~~ t . ~~ ~d ,," 0'" on"> "-~ II ",< • 1 e"-i <j ~'O ~ "'~ i\:" ~ ~ J. P. Hillis: A diving study on Nephrops norvegicus. To count Nephrops it was necessary to extract them from their lmrrows; to estimate their density it was necessary to mark the extent of the area being surveyed. To drive them from their burrows, three methods were initially tried, namely (I) injection of formalin, (2) injection of compressed air, and (3) use of a small diameter bottle·cleaning brush attached to a 1.2 m flexible nylon rod. For injection of formalin a football bladder was used, attached to a 50 em length of aquarium tubing which was closed with a spring clamp when out of use, while the formalin was rendered visible by an admixture of gentian violet. This equipment was carried by the diver in a small string bag. 01' these methods, all demonstrated the extent of a single burrow system, dyed formalin and compressed air emerging from all holes in a burrow·system and particles of mud moved by currents set up by the brush doing likewise. The coloured formalin, however, was found to be the only effective agent in expelling the in· mates and accordingly it was used for all work after the first day. Plate IA shows a Nephrops emerging from its burrow, with two other burrows visible in the right back· ground, the natural darkness of the burrows being enhanced by the presence of dyed formalin therein. Plate IB shows· more clearly the presence of a cloud of formalin, also showing, especially at the top left hand corner of the plate, part of the suspension of mud particles, which makes the sea in this area turbid. These appear as pale spots, rather out of focus. To demarcate a known area on the sea·bed it was considered best for divers to survey an area of con· siderable length, but narrow enough for full examination to be effected by moving along its length once. This avoided difficulties due to clouds of mud stirred up by the divers and departure of N ephrops from parts of the area disturbed prior to being examined. A plot of 1.8 m x 15.2 m was laid out by starting frOOl two weights separated by a 1.8 m line; from each of these weights 15.2 m of line was paid out by a diver swimming away at rightangles to the 1.8 m line paying out 15.2 m by means of two spear·fishing gun reels mounted 1.8 m apart on a tubular steel rod which itself became the fourth side of the rectangular area when the line was fully paid out. The orientation of the long axis of the rectangle was diagonally across the tidal current and the diver swam diagonally against it, so that any silt stirred up by the operation was removed from his field of view and from the plot with maximum speed. For ease of location from the surface, and recovery of the equipment on com· pletion of work, one of the original two weights was linked by a slack line to a heavy weight anchoring a marker buoy. For distribution patterns of burrow systems and the behaviour of N ephrops, including when evicted by formalin and when threatened by the trawl, observations were simply made by the divers and recorded in writing on return to the surface. Mter the day's' diving programme was completed, these observations were discussed and the divers compared their experiences, a process which assisted in eliminating any discrepancies from the preliminary reports. Results The basic type of burrow was found to have a wide·mouthed entrance into a crater with a rampart and another smaller, more vertical entrance, but elaborations were frequent and from one to six entrances were recorded with the addition of a number of very small roughly circular holes without any depression surrounding them and having short roughly vertical shafts. Plate Ie shows a Nephrops in the entrance to its burrow, and illustrates the nature of the crater very clearly. It was noticed that holes of large diameter tended to be more closely grouped than small holes, i.e. they appeared to appertain to shorter burrows or smaller cOOlplexes', pos· sibly because the necessity for protection was less among larger Nephrops. The number of entrances and approximate extent of burrows are given in Table I, showing more than half the burrows at each location to have 3 or more entrances (the over·all percentage was 69%). It is of inter· est to note that the two burrow systems found with more than 4 entrances both harboured two Nephrops, of which one was tiny in each case. Table 2 gives observations on the density of burrows and of N ephrops observed in the survey plots. One of the observations was made in June and the other four in September. The latter emphasises the closeness of the diving area to the margin of the N ephrops area with uninterrupted decrease in numbers in the columns for September from left to right. This evidence implies the confinement of the population to areas of soft substrate . 5 6Zii~'~Dl~rif!i'()tlSeh;ations of Numbe<rs and Distance apart of Entrance, June 1971. ----------------~----------~~---~--Date Position (see Fig, 1) Divers 15 June A (J. 9.5 F 40.9) JB, Be 16 June B (J 9.2 F 38.5) JB, JM Number of Entrances 3 3 2 3 I 2 3 I 3 2 3 3 6 4 2 5 4 2 4 4 4 4 3 Estimated "inter-entrance" distance (em) 36, 28, 18 74, 56, 28 36 38, 7, ? 28 38 137, ?, ? 15 46, 10, ? max. 46 max. 51 max. 56 20 max. 107 15, 15, 15, 15 18 "Very close" max. 33 max. 56 max. 53 ? Summary :- numbers of burrows with N entrances. N 2 3 4 15 June 2 2 5 16 June 3 3 6 Total 2 5 8 6 % 9 22 35 26 -,~--~--General observations Inhabitant Nephrops Ohe small* None None 2 of which 1 tiny* I Other observations "Blowhole" 53 em. away Compact system 2 of which 1 tiny* Long) system 1 Compact system 2 Long system 5 6 4 4 Total ~ 14 33 100 *Approximate carapace lengths of divers' size groupings:- tiny=up to 22 mm; small=23-28 mm; mediuffi=29-34 mm; large=35 mm and Olver. Regarding the action of the trawl, very limited observations indicated a certain variability. On 7 Septem-ber the ends Df the footrDpe Dn the pDrt SIde was seen to' be dlgglllg mto the substrate about one inch and some fish but nO' Nephrops were seen being caught. On 8th September, however, when the height Df the head-hne was estlm~ted at 0.9 m and the footrope was level with it or if anything slightly fDrward, the wing ends were engulfed III such clouds of mud that no details cDuld be seen. Nephrops were seen to dart back from the footrDpe several times in vain attempts to escape. On 16 September, however, when the trawl was observed to be mDvIllg very slowly, the absence of such an escape reaction was noted. Minor behavioural Dbservations noted included, on 14 Jnne, the total absence of visible Nephrops during 18.12 to 18.50 hours, while during 19.18 to 19.45 hours many Nephrops were observed with antennae and chelae visible at their burrow entrances. 6 I. P. Hillis: A diving study on N ephrops norvegicus. Table 2. Density of Nephrops and Burrows. Date Position (Fig. 1) Decca reading Diver Substrate type Area dimensions (m) Area (m2) Number of Nephrops {tiny small medium 4 large Total Number of burrows Area (m2) per Nephrops Area (m2) per burrow 15 June A J9.5 F40.9 Be, IM 14.6 x 2.4 35.6 4 9 8.9 4.0 16 Sept. 1 I8.05 F41.45 Be Soft 15.2 x 1.8 27.9 I 6 4 11 14 2.5 2.0 16 Sept. 2 J8.05 F41.78 SS Fine, soft 15.2 x 1.8 27.9 3 4 7 13 4.0 2.1 16 Sept. 3 J8,OS F42.08 JB Fine, soft 15.2 x 1.8 27.9 I 3 2 6 9 5.6 3.1 16 Sept. 4 I8.05 F42.38 IM quite hard 15.2 x 1.8 27.9 3 9.3 (Ohapman & Rice, 1971) 14 x 10 approx. 140 2 I 14 17 66 7.8 2.1 ------------------------_._----_._-----BehaviDur Dn eviction from burrowsl by formalin included a case when introduction of formalin at one entrance caused the inhabiting Nephrops to emerge from a second hole and go calmly and without hurry to a third which it entered backwards, b,ut on encountering formalin there it became extremely aggressive and agi-tated and departed from the area at great speed, thereby indicating the extent to which a choice of entrances is used as an aid to' safety, Animals making burrows similar to those of N ephrops Were limited to a very few Gonep/ax rhomboides and it is considered that misidentification of untenanted burrows, whilst not impossible, occurred only very seldom if at all. Dicussion The pDwerful backward propulsion of the tail and large chelae of the Astacidea appear well adapted for escape into and defence of entrances of cavities, and in the case of Nephrops the animals, create their own sanctuary. In captivity, they prefer an artificial refuge to the open aquarium. The practice of excavating bur-rows is shared, in appropriate habitats, by the American lobster Homarus americanus (Cobb, 1971). In their extensive, field studies on burrowing behaviour and bun-ow form in Loch Torridon, Chapman and Rice (1971) and Rice and Chapman (1971) found densities of burrows approximately similar to those found in the present work. The most basic form of bnrrow has a large entrance at one end opening off a crater sur-rounded by a rampart and a small nearly vertical aperture at the other. However, a very large proportion of burrows in the present investigation had more than two entrances, 69% (Table 1) as opposed to 8% in 1968 and 20% in 1969 in that of Rice and Chapman; in the present studies, three was the most frequent number and up to' six was recorded. Thisl difference appears remarkable, but is probably explained by an observation of Dybern and Hoisaeter, who note that in the aquarium homeless Nephrops try to enter existing burrows, regard-less of whether they are already occupied, and if not ejected proceed to make branches and new entrances. It therefore follows that the average burrow system would be expected to be more oomplex in areas where fre-quent trawling produces mnch disturbance of the popUlation, such as the Irish Sea, than in Loch Torridon, where trawling is not carried out on a large scale, if at all. The impression gained in the present work of inverse relationship between burrow length and bnrrow diameter (the latter indicating the size of the inhabitant) is at variance with the findings of Dybern and Hoisaeter: they found a rather consistent positive correlation be-tween Nephrops length (above about 27 mm Lc) and burrow length. It may be that the effect observed in the present work is limited to carapace lengths below 27 mm. 7 Irish Fisheries Investigations Series B, No. 12 (1974). Acknowledgment I wish to acknowledge the assistance received from Mr. John Browne of the Department of Agriculture and Fisheries. Mr. ~Browne, who is a qualified diver, evolved the underwater techniques used and directed all as-pects of work on the sea-bed. My thanks also go to the other divers who participated, Messrs. B. Cusack, J. Murray, L. Clegg, W. Crowley, S. Sheridan and C. Craig, frequently wOTking in conditions of depth and water turbidity considerably severer than those to which they are accustomed. Fig. 1 represents, the coast of Ireland by permission of the Ordnance Survey, Permit No. 1833 and nautical data are taken from the Admiralty chart with overlay grid of the Navigational Aid system of Messrs. Decca, Ltd. References Chapman, D. J. and Rice, A. L. (1971). Some direct observations on the ecology and behaviour of the Norway lobster, Nephrops norvegicus. Marine Biology 10, 321-329. Cobb, J. S. (1971). The shelter-related behaviour of the lobster, Homarus americanus. Ecology. 52, 108-113. Cmkovic, D. (1968). Some observations regarding the burrows of juvenile Nephrops norvegicus (L.). Rapp. P.-v. Reun. Commn. into Explor. scient. Mer. Mediterr. 19, 171-172. Dybem, B. I., and Hoisaeter, T. (1965). The burrows of Nephrops norvegicus (L.). Sarsia 21, 49-55. Gauss-Garady, V. (1913). Dber die Lebensgeschichte des adriatischen Scamp; (Nephrops norvegicus). Ost. Fisch. Ztg. 9, 42-44, 61-63, 77-79. Hillis, J. P. (1971). Effects of Ught on Nephrops catches. ICES. CM 1971 Doc. No. K2 (mimeo). Hillis, J. P~. (1972). Studies on the Biology and Ecology at the Norway Lobster Nephrops norvegicus (L.) in Irish Waters. Ph.D. Thellis, unpublished, National University of Ireland. Hoglund, H. (1942). Havskraftan eller Kejsarhummen, Nephrops norvegicus. Andersson K.A.ed. Fiskar Fiske 1, 293-296. Priestley, R. and Thomas, H. J. (1964). The burrowing habit of Nephrops norvegicus L. ICES C.M. 1964. Shellfish Cttee. Doc. No. 148 (Mimoo). Rice, A. L. and Chapman, C. J. (1971). Observations on the burrows and burrowing behaviour of two mud-dwelling decapod crustaceans, Nephrops norvegicus and Goneplax rhomboides. Marine Biology. 10, 330-342. 8 At Bt ct Plate 1. A-Nephrops emerging from its burrow, the darkness of which is accentuated by the presence of dyed formalin driving the animal out. B-A cloud of dyed formalin drifting out of a burrow entrance, Pale, poorly fccllssed SPGts at top left are particles of sediment suspended in the water. C-Nephrops in its burrow entrance, showing clearly the form of the sunken 'crater' at the entrance. 9 Y;~t:+lill:ii~iJ~6{4f'4irffll()f4!i1i/'?IrIPBcirlmthe~st of Co. Kerry. r~:" @n,ifun',:,':{/;-:,"J<//',', '«0;;:,';'> ':«:-<, .:,:-«r<>:<» '<;' "-" " l\plll*i~:;911th~.oc!Cyshores ofB!lntry Bay, i~~~~(li(j,iagl!~taL,) .ClIught ~ rod lind line .00 the. ~~~~~ft~l~i~;~~~fi~'~I~~~rQ'P;.fJ<Jr~f'gicus. (L.).lIndttieirburtmysolf th" .Cl\St 

A diving study on Dublin Bay prawns Nephrops norvegicus (L) and their burrows off the east coast of Ireland

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A diving study on Dublin Bay prawns Nephrops norvegicus (L) and their burrows off the east coast of Ireland

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