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Larvae of the Dublin Bay prawn Nephrops norvegicus (L.) in Galway Bay, Ireland
Estuarine and Coastal Marine Science (1976) 4, 225-229
Notes and Discussions Larvae of the Dublin Bay Prawn Nephrops norvegicus (L.) in Galway Bay, Ireland Padraic de Bhaldraithe Department of Oceanography, University College, Galway, Ireland Received I2June 1975 and in revised form I4jdy 1975
A quantitative study of the distribution of the larvae of Nephrops norvegicus in Galway Bay showed a larval seasonfrom February to May, with greatest hatching in the March-April period. Maximum larval density was at 20 m by day. It is suggested that a salinity layering pattern may cause their retention in the bay.
Introduction The larval stages of Nephrops were described by Sars (1884, 1889) Williamson (1915), Jorgensen (1925) and Santucci (1926, 1927). Three stages are recognized-referred to as stages I, II and III. Figueiredo & Thomas (1967) stated that in the Adriatic and Tyrrhenhian Seas the larval season extends from January to March, but that in northern waters hatching does not start until April. Data on the distribution and changes in stage composition of the larvae were scarce until Hillis (1974) examined these changes in the western Irish Sea and found that the main larval season extended from middle or late April until late June, or possibly later. He found that the depth of occurrence of maximum numbers was about 10-15 fathoms (18-27.5 m) by day over most of the season, tending to ascend towards the surface by about 5 fathoms (9 m) at dusk. Very few larvae inhabited depths greater than 20 fathoms until late in the season when they became numerous at 20, 30 and 40 fathoms. No firm indications of the movements of larvae emerged from this study. In Galway Bay, where the occurrence of adult populations of Nephrops is well documented (O’Riordan, 1964), there is very little information on larval distribution, previous plankton surveys reporting Nephrops larvae as rare (Carr, 1962) or absent (O’Ceidigh, 1962; O’Riordan, 1964). Present work, based on analyses of plankton samples taken by the Department of Oceanography, University College Galway, since 1972, gives limited but quantitative data on the distribution of Nephrops larvae in relation to the Galway Bay adult population of this commercially important prawn.
hiaterials and methods Galway Bay, on the west coast of Ireland, is bounded by the lines of longitude of 8”55’W and 9’5o’W and lies between latitudes of 53”oo’N and 53’15’N. The two main channels into the bay are the North Sound and South Sound, smaller channels being the Gregory and Foul Sounds (Figure I). Bottom depths are up to 30 m in the inner bay (east of 9”16’W) and up to 60 m in the outer bay. Plankton samples were taken at selected stations throughout the bay using opening-closing samplers with a mouth diameter of 12.3 cm and a mesh 2%
226
P. de Bhaldraithe
aperture of 0.4 mm2 (Clarke & Bumpus, 1950). Up to four depths were sampled at each station and the volume of water filtered was estimated using a built-in flowmeter. The samplers were towed horizontally for 15 min, filtering in the order of 10-15 m3 of water each. All sampling was carried out during daylight hours. Some samples were also taken with a ring net of 0.5 m diameter and a mesh aperture of 0.28 mm2. The Nephrops grounds in the Galway Bay area (Figure I) are described by O’Riordan (1964, Figure 3). They are situated off the north shore of the bay, west of Spiddal, are about one mile wide and run east-west for about seven miles. Bottom depths range between 25 and 35 m. Further grounds exist to the south-west of the Aran Islands and in the North Sound, but neither is believed to be extensive.
North
Sound
South
Sound
0 Figure I. Map of the Galway Nephrops grounds.
53O Co Clare 90
Bay area. The shaded area represents
the main
Results Table I shows the number of larvae found and their stage composition during the breeding seasonsof 1973, 1974 and 1975. First stage larvae appeared in February 1974 and in March in 1975. The three larval stages were found simultaneously in April only. No first stage larvae were found after April except for one in May 1975. One post-larva was taken in a sample from 15 m a few miles west of Spiddal in April 1975. The first stage larvae were found in the inner bay; they occurred in smaller numbers in the outer bay and South Sound and were quite scarce in the North Sound. The later stages
Larvae of N. norvegicus in Galway Bay
227
followed the same pattern of distribution. Figure z illustrates the larval distribution in April 1975 when larval concentration was up t/ma of water. TABLE I. Number Year I973
1974
1975
Month March April May February March April May February March April May
of Nephrops larvae and their stage composition. Week
Stage II
I
2
IO
-
3
13
17
2
-
-
2 2
-
-
7 21
-
-
;
-
-
6
I3
3 2
III
I
-
2
37
-
3 2
II I
41
5 -
32
-
Table 2 shows the average concentration of the larval stages at the depths sampled. No second or third stage larvae were found in the surface samples and all stages showed a general increase in concentration towards 20 m. Decrease in concentration below 20 m may be attributed to their lower density in outer bay and to the smaller number of samples in the 25-50 m range.
Figure 2. Average concentration of Nephrops larvae (all stages) in Galway Bay, April 1975. 0, Plankton station with larvae present; @, Plankton station with larvae absent. E, up to o-5/10 ma; 1111, 0.5-I.O/IO ma; $:, 1.5110 m*.
228
P. de Bhaldraithe
2. Average concentration
TABLE
Numbers Depth 0
5 IO
I 0’7 2’3 2.9
15 20
63:;
25
2’7 0’0 0.0 0’0
30 35 40 45 50
0’0
per II
of larvae at 5 m depth intervals IOO rn’
III
0’0 3’4
0’0 I’7
2’0
2’0
5’2 7’2 0’0 6.0 0’0 0’0 0’0
2.7 3.6 0’0 0’0 0’0 2’1
0’0
Discussion Data on the seasonal and spatial distribution of Nephrops larvae are particularly valuable in view of the lack of data on berried females and postlarvae. The spatial distribution of larvae in coastal areas is also of interest in understanding the mechanism of recruitment to the adult stock. In Galway Bay the hatching of larvae begins in late February to early March and finishes by late April or early May. This is approximately two months earlier than in the Irish Sea and is similar to the hatching season in more southerly areas. During a nine-day survey in May 1974 of Killary Harbour, a fjord-like bay in Co. Mayo, west of Ireland, plankton samples were taken throughout the area and no larvae were found, yet this bay contains an adult population of Nephrops. In Galway Bay the distribution of larvae is concentrated in the inner bay, with maximum concentration by day at 20 m in an area eastwards of the adult population. Hillis (1974) measured changes in illumination in connection with diurnal vertical migration of the larvae and found a maximum concentration of larvae at night in the S-IO fathom (9-18 m) range. On the basis of light being the main regulatory factor, one would expect the maximal concentration of larvae at night in Galway Bay to be in the same range. Thus over a q-hour period maximum larval density would be within a range of 9-27'5 m. In a plain estuary such as the bay, which is affected by freshwater outflows, one would expect a net inflow of water in the lower part of the water column (Dyer, 1972). During the spring months in Galway Bay, at a time of increased freshwater outflow, a pronounced horizontal salinity layering has been shown to be present. Their vertical distribution in the water column may therefore be responsible for completion of the larval development of most Nephrops within areas such as Galway Bay.
References Carr, J. 1962 Studies in the inshore plankton of Galway Bay; A Survey of the Decapod Larvae in the Plankton off Mutton Island, November 1961 to August 1962. Thesis presented to the National University of Ireland for the Studentship in Zoology. Clarke, G. L. & Bumpus, D. F. 1950 The plankton sampler. An instrument for quantitative plankton investigations. American Society of Limnology and Oceanography, Special Publication No. 5. Dyer, K. C. 1972 Estuaries: A Physical Introduction. Wiley Interscience Publication, Chichester and New York. de Figueiredo, M. J. & Thomas, H. J. 1967 Nephrops nomegicus (Linnaeus 1758) Leach-a review. Oceanography and Marine Biology-An Annual Review. Volume 5, pp. 371-408.
Larvae
Hillis,
of N. norvegicus
in Galway
Bay
229
J. P. 1974 Field observations on larvae of the Dublin Bay Prawn, Nephrops nomegicus (L.) in the western Irish Sea. Irish Fisheries Investigations Series B, No. 13. Jorgensen, 0. M. rgzs The early stages of Nephrops norwegicus from the Northumberland plankton together with a note on the post-larval development of Homarus vulgaris. Journal of the Marine Biological Association of the United Kingdom (New Series) 13, 870-879. O’Ceidigh, P. 1962 The Marine Decapoda of the Counties Galway and Clare. Proceedings of the Royal Irish Academy 62, (B) No. I I. O’Riordan, C. 1964 Nephrops norwegicus the Dublin Bay Prawn in Irish Waters. Scientific Proceedings of the Royal Dublin Society Series B.r, pp, 131-157. Santucci, R. xgz6 Lo sviluppo e l’ecologia post embrionale dello ‘ Scampo’ (Nephrops norvegicus (L.)) nel Tirreno e nei mari nordici. Relazioni Comitato talassografico italiano. Memoria 125. Santucci, R. 1927 Uno stadio di sviluppo non ancora descritto dello ‘Scamp0 Nephrops nomegicus (L.) Relazioni Comitato talassografico italiano. Memoria 128. Sars, G. 0. 1884 Bidrag til Kundskaben om Dekapodernes Forvandlinger I. Archiv for mathematik og naturvidenskab. Kristiania 9, 155-204. Sars, G. 0. 1889 Bidrag til Kundskaben om Dekapodemes Forvandlinger II. Archiv for mathematik og naturvidenskab. Kristiania 13, 133-201. Williamson, H. C. rgrs Nordisches Plankton. Band III (Malacostraca) VI Crustacea Decapoda, Laroen. pp. 133-588. Neudruck A. Asher & Co., Amsterdam.
Notes and Discussions Larvae of the Dublin Bay Prawn Nephrops norvegicus (L.) in Galway Bay, Ireland Padraic de Bhaldraithe Department of Oceanography, University College, Galway, Ireland Received I2June 1975 and in revised form I4jdy 1975
A quantitative study of the distribution of the larvae of Nephrops norvegicus in Galway Bay showed a larval seasonfrom February to May, with greatest hatching in the March-April period. Maximum larval density was at 20 m by day. It is suggested that a salinity layering pattern may cause their retention in the bay.
Introduction The larval stages of Nephrops were described by Sars (1884, 1889) Williamson (1915), Jorgensen (1925) and Santucci (1926, 1927). Three stages are recognized-referred to as stages I, II and III. Figueiredo & Thomas (1967) stated that in the Adriatic and Tyrrhenhian Seas the larval season extends from January to March, but that in northern waters hatching does not start until April. Data on the distribution and changes in stage composition of the larvae were scarce until Hillis (1974) examined these changes in the western Irish Sea and found that the main larval season extended from middle or late April until late June, or possibly later. He found that the depth of occurrence of maximum numbers was about 10-15 fathoms (18-27.5 m) by day over most of the season, tending to ascend towards the surface by about 5 fathoms (9 m) at dusk. Very few larvae inhabited depths greater than 20 fathoms until late in the season when they became numerous at 20, 30 and 40 fathoms. No firm indications of the movements of larvae emerged from this study. In Galway Bay, where the occurrence of adult populations of Nephrops is well documented (O’Riordan, 1964), there is very little information on larval distribution, previous plankton surveys reporting Nephrops larvae as rare (Carr, 1962) or absent (O’Ceidigh, 1962; O’Riordan, 1964). Present work, based on analyses of plankton samples taken by the Department of Oceanography, University College Galway, since 1972, gives limited but quantitative data on the distribution of Nephrops larvae in relation to the Galway Bay adult population of this commercially important prawn.
hiaterials and methods Galway Bay, on the west coast of Ireland, is bounded by the lines of longitude of 8”55’W and 9’5o’W and lies between latitudes of 53”oo’N and 53’15’N. The two main channels into the bay are the North Sound and South Sound, smaller channels being the Gregory and Foul Sounds (Figure I). Bottom depths are up to 30 m in the inner bay (east of 9”16’W) and up to 60 m in the outer bay. Plankton samples were taken at selected stations throughout the bay using opening-closing samplers with a mouth diameter of 12.3 cm and a mesh 2%
226
P. de Bhaldraithe
aperture of 0.4 mm2 (Clarke & Bumpus, 1950). Up to four depths were sampled at each station and the volume of water filtered was estimated using a built-in flowmeter. The samplers were towed horizontally for 15 min, filtering in the order of 10-15 m3 of water each. All sampling was carried out during daylight hours. Some samples were also taken with a ring net of 0.5 m diameter and a mesh aperture of 0.28 mm2. The Nephrops grounds in the Galway Bay area (Figure I) are described by O’Riordan (1964, Figure 3). They are situated off the north shore of the bay, west of Spiddal, are about one mile wide and run east-west for about seven miles. Bottom depths range between 25 and 35 m. Further grounds exist to the south-west of the Aran Islands and in the North Sound, but neither is believed to be extensive.
North
Sound
South
Sound
0 Figure I. Map of the Galway Nephrops grounds.
53O Co Clare 90
Bay area. The shaded area represents
the main
Results Table I shows the number of larvae found and their stage composition during the breeding seasonsof 1973, 1974 and 1975. First stage larvae appeared in February 1974 and in March in 1975. The three larval stages were found simultaneously in April only. No first stage larvae were found after April except for one in May 1975. One post-larva was taken in a sample from 15 m a few miles west of Spiddal in April 1975. The first stage larvae were found in the inner bay; they occurred in smaller numbers in the outer bay and South Sound and were quite scarce in the North Sound. The later stages
Larvae of N. norvegicus in Galway Bay
227
followed the same pattern of distribution. Figure z illustrates the larval distribution in April 1975 when larval concentration was up t/ma of water. TABLE I. Number Year I973
1974
1975
Month March April May February March April May February March April May
of Nephrops larvae and their stage composition. Week
Stage II
I
2
IO
-
3
13
17
2
-
-
2 2
-
-
7 21
-
-
;
-
-
6
I3
3 2
III
I
-
2
37
-
3 2
II I
41
5 -
32
-
Table 2 shows the average concentration of the larval stages at the depths sampled. No second or third stage larvae were found in the surface samples and all stages showed a general increase in concentration towards 20 m. Decrease in concentration below 20 m may be attributed to their lower density in outer bay and to the smaller number of samples in the 25-50 m range.
Figure 2. Average concentration of Nephrops larvae (all stages) in Galway Bay, April 1975. 0, Plankton station with larvae present; @, Plankton station with larvae absent. E, up to o-5/10 ma; 1111, 0.5-I.O/IO ma; $:, 1.5110 m*.
228
P. de Bhaldraithe
2. Average concentration
TABLE
Numbers Depth 0
5 IO
I 0’7 2’3 2.9
15 20
63:;
25
2’7 0’0 0.0 0’0
30 35 40 45 50
0’0
per II
of larvae at 5 m depth intervals IOO rn’
III
0’0 3’4
0’0 I’7
2’0
2’0
5’2 7’2 0’0 6.0 0’0 0’0 0’0
2.7 3.6 0’0 0’0 0’0 2’1
0’0
Discussion Data on the seasonal and spatial distribution of Nephrops larvae are particularly valuable in view of the lack of data on berried females and postlarvae. The spatial distribution of larvae in coastal areas is also of interest in understanding the mechanism of recruitment to the adult stock. In Galway Bay the hatching of larvae begins in late February to early March and finishes by late April or early May. This is approximately two months earlier than in the Irish Sea and is similar to the hatching season in more southerly areas. During a nine-day survey in May 1974 of Killary Harbour, a fjord-like bay in Co. Mayo, west of Ireland, plankton samples were taken throughout the area and no larvae were found, yet this bay contains an adult population of Nephrops. In Galway Bay the distribution of larvae is concentrated in the inner bay, with maximum concentration by day at 20 m in an area eastwards of the adult population. Hillis (1974) measured changes in illumination in connection with diurnal vertical migration of the larvae and found a maximum concentration of larvae at night in the S-IO fathom (9-18 m) range. On the basis of light being the main regulatory factor, one would expect the maximal concentration of larvae at night in Galway Bay to be in the same range. Thus over a q-hour period maximum larval density would be within a range of 9-27'5 m. In a plain estuary such as the bay, which is affected by freshwater outflows, one would expect a net inflow of water in the lower part of the water column (Dyer, 1972). During the spring months in Galway Bay, at a time of increased freshwater outflow, a pronounced horizontal salinity layering has been shown to be present. Their vertical distribution in the water column may therefore be responsible for completion of the larval development of most Nephrops within areas such as Galway Bay.
References Carr, J. 1962 Studies in the inshore plankton of Galway Bay; A Survey of the Decapod Larvae in the Plankton off Mutton Island, November 1961 to August 1962. Thesis presented to the National University of Ireland for the Studentship in Zoology. Clarke, G. L. & Bumpus, D. F. 1950 The plankton sampler. An instrument for quantitative plankton investigations. American Society of Limnology and Oceanography, Special Publication No. 5. Dyer, K. C. 1972 Estuaries: A Physical Introduction. Wiley Interscience Publication, Chichester and New York. de Figueiredo, M. J. & Thomas, H. J. 1967 Nephrops nomegicus (Linnaeus 1758) Leach-a review. Oceanography and Marine Biology-An Annual Review. Volume 5, pp. 371-408.
Larvae
Hillis,
of N. norvegicus
in Galway
Bay
229
J. P. 1974 Field observations on larvae of the Dublin Bay Prawn, Nephrops nomegicus (L.) in the western Irish Sea. Irish Fisheries Investigations Series B, No. 13. Jorgensen, 0. M. rgzs The early stages of Nephrops norwegicus from the Northumberland plankton together with a note on the post-larval development of Homarus vulgaris. Journal of the Marine Biological Association of the United Kingdom (New Series) 13, 870-879. O’Ceidigh, P. 1962 The Marine Decapoda of the Counties Galway and Clare. Proceedings of the Royal Irish Academy 62, (B) No. I I. O’Riordan, C. 1964 Nephrops norwegicus the Dublin Bay Prawn in Irish Waters. Scientific Proceedings of the Royal Dublin Society Series B.r, pp, 131-157. Santucci, R. xgz6 Lo sviluppo e l’ecologia post embrionale dello ‘ Scampo’ (Nephrops norvegicus (L.)) nel Tirreno e nei mari nordici. Relazioni Comitato talassografico italiano. Memoria 125. Santucci, R. 1927 Uno stadio di sviluppo non ancora descritto dello ‘Scamp0 Nephrops nomegicus (L.) Relazioni Comitato talassografico italiano. Memoria 128. Sars, G. 0. 1884 Bidrag til Kundskaben om Dekapodernes Forvandlinger I. Archiv for mathematik og naturvidenskab. Kristiania 9, 155-204. Sars, G. 0. 1889 Bidrag til Kundskaben om Dekapodemes Forvandlinger II. Archiv for mathematik og naturvidenskab. Kristiania 13, 133-201. Williamson, H. C. rgrs Nordisches Plankton. Band III (Malacostraca) VI Crustacea Decapoda, Laroen. pp. 133-588. Neudruck A. Asher & Co., Amsterdam.