Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: 70-84. 1989

Spatial patterns in a coastal ichthyoplankton community southwest of Ireland

Miriam J. Doyle and Thomas A. Ryan

Doyle, Miriam J., and Ryan, Thomas A. 1989. Spatial patterns in a coastal ich­ thyoplankton community southwest of Ireland. - Rapp. P.-v. Réun. Cons. int. Explor. Mer, 191: 70-84.

Plankton and oceanographic sampling were carried out in two bays, and adjacent waters, off the southwest Irish coast during April and June 1979. Patterns in distri­ bution and abundance of ichthyoplankton and zooplankton species were documented and examined in relation to spatial variation in the physical environment. Analysis of spatial patterns in the data was enhanced using the multivariate technique of numerical classification. The ichthyoplankton distribution patterns observed reflect principally the spawning patterns of the adult fish. A seasonal, seaward progression in spawning activity is apparent among the fish species encountered and it is suggested that this may be related to spatial variation in both water temperature and plankton production. The distribution patterns of most ichthyoplankton species are remarkably similar to those observed among many of the zooplankton species, particularly copepods. It is postulated that patterns of zooplankton distribution in these coastal waters may influence the spawning behaviour of the adult fish, and perhaps sub­ sequently larval behaviour, resulting in a high level of spatial agreement between the occurrence of fish larvae and their most important food organisms, copepods.

Miriam J. Doyle' and Thomas A. Ryan2; Department o f Oceanography, University College, Galway, Ireland.

Introduction An oceanographic and plankton sampling programme undertaken by the Department of Oceanography, Uni­ Hjort (1914) was first to postulate that the strength of versity College, Galway, during 1972 to 1979, provided incoming year classes to adult marine fish populations the opportunity for a small-scale investigation of larval is essentially determined by variations in the rates of fish ecology in coastal waters west of Ireland (Doyle, mortality of their larvae. Factors affecting larval fish 1988). The aim of the study was to identify environ­ survival, and ultimately levels of recruitment to adult mental factors which may influence fish spawning stocks, may include variable egg production, predation patterns, and subsequent distribution, abundance and of eggs and larvae, larval starvation, abiotic and biotic survival of their larvae, in these coastal waters. This features of the marine environment that affect general paper deals with the investigation of ichthyoplankton plankton production, larval transport and the oceano­ distributions in Bantry and Long Island Bays, off the graphic factors that cause it, interspecific competition, southwest coast, in relation to physical and biological and the effect of localized and widespread oceano­ features of the pelagic environment. graphic events (Lasker, 1985). Research in the area of larval fish ecology is recognized, therefore, as being vital to understanding natural fluctuations in fish popu­ Methods and materials lations. Consideration of the position of larval fish in an ecosystem and the patterns of distribution among larval Sampling procedure fish species, their food and predators, is now thought Oceanographic and plankton sampling were carried out to be of fundamental importance in investigations of at 18 stations in Bantry and Long Island Bays, and the larval fish survival (Smith and Lasker, 1978; Hunter, adjoining region, once during both April and June 1979 1981; Hewitt, 1981; Lasker, 1981; Frank and Leggett, (Fig. 1). Depths in Bantry Bay range from 30 m off 1983; Ellertsen et al., 1984; Paulsen, 1985). Whiddy Island, at the head of the Bay, to 100 m south 1 Present address: Northwest & Alaska Fisheries Center, 7600 of Dursey Island. In inner Long Island Bay, depths are Sand Point Way NE, Seattle, WA 98115, USA. predominantly less than 30 m and increase to 60 m in 2 Present address: Seaqueen Salmon Ltd., Westport, Co. the outer section. Freshwater influence in these bays is Mayo, Ireland. slight.

70 Whidd Islanc 14 Dursey Islarud

Outer Bantry Bay

51 30 ' 7»

•2 Fastnet

Figure 1. Sampling location and position of 9° 30 ‘ sampling stations.

At each station, water samples were collected and Analysis of spatial patterns in the data was enhanced temperatures measured at two to seven depths using using the multivariate technique of numerical classifi­ NIO water bottles to which reversing thermometers cation. This analytical technique involves grouping simi­ were attached. Salinities were determined using an lar entities together based on numerical data such as Auto-Lab Salinometer. Winkler’s method was used for species abundance in a range of samples (Clifford and the preservation and subsequent analysis of water Stephenson, 1975; Boesch, 1977; Gauch, 1982). Normal samples for dissolved oxygen content (Strickland and and inverse classifications were carried out on the data Parsons, 1972). sets, i.e. both the samples and variables (physical and Zooplankton sampling was carried out at up to four biological) were classified into groups. Only the domi­ discrete depths using Clarke-Bumpus Plankton Sam­ nant ichthyoplankton and zooplankton taxa were sub­ plers (Clarke and Bumpus, 1950). The mouth diameter jected to this analysis. Prior to analysis, the data were of these samplers was 12.5 cm and they were fitted with log-transformed. In the classification procedure, the a calibrated flowmeter and monofilament nylon net with Bray-Curtis measure of dissimilarity (Bray and Curtis, mesh of 0.342 |un square. Horizontal tows of 15 min 1957) was the correlation coefficient used, followed duration were made at a ship speed of approximately by an agglomerative, polythetic, hierarchical clustering 2kt. Plankton samples were fixed in a formalin-sea- method - “Flexible Sorting” (Lance and Williams, water mixture (4—5% formalin), buffered with borax. 1967). These numerical classifications were performed on a Digital Dec2060 computer system, using the Cluster Analysis Package CLUSTAN (Wishart, 1978). Interpretation of the numerical classification results Data analysis was furthered by the application of a post-clustering Standard procedures for identification and enumeration procedure termed nodal analysis (Boesch, 1977). This of ichthyoplankton and zooplankton specimens were was used to compare sample classifications with variable used in the laboratory processing of the plankton classifications and thus characterize the sample groups samples (Ryan, 1982; Doyle, 1988). Levels of abun­ according to physical and biological parameters. A stat­ dance for each species/taxon are expressed as number of istical F-test was also applied to the data, after classifi­ specimens m-3. Abundance data for individual copepod cation, in order to investigate the contribution of species consist almost entirely of combined numbers of different variables to the sample classification. The F- adults and copepodite stages. Nauplii were rare in the statistic was determined by calculating the ratio of vari­ samples collected (Ryan, 1982) and therefore do not ance among sample groups to variance within sample contribute significantly to overall levels of abundance. groups, for each variable.

71 ;8Cr 8 0 8-1 8 O 8 1 7.9. 20 ■7-9' 20-

8-2 40 8-2 40- 8-3 8-3'

60 - 60-

BANTRY BAY BANTRY BAY - PÀSTNET LONG ISLAND BAY

80- Vertical Profiles of Temperature (°C) along station transects.

STN. 18

-3 4 9 40- - 3 5 0

BANTRY BAY BANTRY6 ÀY - PASTNET LONG ISLAND BAY f.

______d. Vertical Profiles of Salinity (S) along station transects.

Figure 2. Vertical profiles of temperature (°C) and salinity (S) during April.

tToT 120. 10-5^ •110 20 - -1 0 -o 9 -7. 40 * 9-5

60* 9 0 BANTRY BAY BANTRY BAY - FASTNET L9NS 'SL.AMD Bay 80 - Vertical Profiles of Temperature (°C) along station transects.

STN. m

34 • 34;95 -, 34-9. .34-9 20- 3 5 0 20 ‘34-9‘ “ 34-95. 350 40- 40 35-1

60- 60

BANTRY BAY BANTRY BAY - FASTNET Q. LONG ISLAND BAY 80- Ve. Sal

Figure 3. Vertical profiles of temperature (°C) and salinity (S) during June. Results show that water of lowest salinity (34.17-34.70 ppt) was confined in its occurrence to inner Bantry Bay and Physical measurements the innermost station in Long Island Bay (Fig. 2d-f). Salinity values of 34.80-35.04 ppt were recorded for During April, a narrow range of temperature and sal­ most samples and represented the water which occurred inity values was recorded in the sampling area. The in outer Bantry Bay, along the Bantry-Fastnet transect narrow range of temperatures (7.8-8.4°C) is typical and throughout most of Long Island Bay. of spring conditions. Temperature profiles show that Due to seasonal warming of the upper layers of water highest temperatures (8.1°C) were recorded at depths and a significant level of freshwater runoff in the inner­ of 40 m and below in outer Bantry and outer Dunmanus most parts of Bantry and Long Island Bays, a larger Bays, at stations 11, 12, and 18 (Fig. 2a-c). Salinities range of both temperature and salinity values was re­ ranged from 34.17 to 35.04 ppt and the salinity profiles corded during June than in April. Throughout most of the sampling area, temperatures ranged from 9-9.7°C near the bottom to 11-12°C at the surface (Fig. 3a-c). In the upper 5-10 m of water in inner Bantry Bay, Table 1. List of larval fish taxa recorded off the southwest coast however, highest values of 12-16°C were recorded. during April and June of 1979. Conversely, lowest salinities of 33.33-34.14 ppt were Family Clupeidae recorded in this region (Fig. 3d-f). Below 10 m in Clupea harengus Bantry Bay, from Bantry to Fastnet and throughout Sprattus sprattus most of Long Island Bay, salinities ranged from 34.80 Family Argentinidae Argentina sphyraena to 35.10 ppt. Family Syngnathidae Enteleurus aequoreus Family Gadidae Ichthyoplankton species composition, levels of Trisopterus luscus abundance, and patterns in distribution Trisopterus spp. Merlangius merlangus A list of the families and species of fish larvae recorded Mol va molva during the present study is given in Table 1. Apart from Rhinonemus cimbrius the genus Trisopterus spp. and the family Gobiidae, Ciliata mustela for which species identification is problematic (Russell, Family Labridae Labrus mixtus 1976), a total of 31 species of fish larvae were identified. Labrus bergylta Fish eggs were identified to species for April samples Crenilabrus melops only; they belong almost exclusively to the species Family Ammodytidae Sprattus sprattus. Gymnammodytes semisquamatus Almost all the larvae caught were less than 10 mm Hyperoplus lanceolatus Hyperoplus immaculatus and predominantly 3-7 mm in length, suggesting that Ammodytes to bianus they were early feeding larvae. Levels of relative abun­ Family Gobiidae dance for the dominant larval fish taxa, during both Family Callionymidae sampling periods, are given in Table 2. Nine taxa, Callionymus lyra Family Blennidae Blennius pholis Family Pholidae Pholis gunnellus Table 2. Levels of relative abundance among the dominant Family Cottidae taxa of fish larvae recorded off the southwest coast during Taurulus bubalis April and June of 1979. Calculations are based on abundance Taurulus lilljeborgi data (no. rrr3). Values represent the percentage of total larval Family Liparidae fish abundance accounted for by each taxon in each sampling Liparis montagui period. - = not recorded. Family Bothidae Zeugopterus punctatus April June Phyrnorhombus norvegicus Taxon % % Family Pleuronectidae Limanda limanda Sprattus sprattus 22.00 60.20 Microstomus kitt Ammodytes tobianus 47.00 <1.00 Glyptocephalus cynoglossus Gymnammodytes semisquamatus 11.00 6.82 Platicthys flesus Callionymus lyra _ 7.30 Family Soleidae Gobiidae 2.00 7.28 Buglossideum luteum Microstomus kitt _ 3.06 Microchirus variegatus Trisopterus spp. - 2.90 Family Gobiesocidae Phyrnorhombus norvegicus _ 2.53 Lepadogaster candollei Limanda limanda _ 1.87

73 including seven species, account for over 80% of the all patterns of ichthyoplankton distribution being exam- fish larvae recorded. These abundant larvae are the ined here. offspring of spring-spawning species of fish including During April, diversity and abundance levels of larval sprat, Trisopterus spp., the dragonet Callionymus lyra fish species were low (Table 2, Fig. 4). A total of only and some species of sand-eel, gobies, and flatfish. The 10 species and the family Gobiidae were recorded and remaining larval fish species accounted for less than 80% of total larval fish abundance was accounted for 1% of total larval fish abundance recorded on either by only three species: Sprattus sprattus (22%) and the occasion and do not contribute significantly to the over- sand-eels Ammodytes tobianus (47%) and Gymnam-

0 •

Abundance Scale :

N a m 3 o - 0 • 0-1 • 0-3 • > 0-5 • > 1-0 • > 3 0 • > 5-0 • > 10-0 • > 30-0 • > 50-0 Figure 4. Distribution of sprat eggs and dominant larval fish species during April. Abundance expressed as mean no. rrT3 (averaged over different sampling depths) at each station, a. Sprattus sprattus (eggs), b. Sprattus sprattus (larvae), c. Am m odytes tobianus, d. Gymnammodytes semisquamatus.

74 O'

7 ^

x=’- &

Figure 5. Distribution of fish eggs and dominant larval fish species during June. Abundance expressed as mean no. m~3 (averaged over different sampling depths) at each station. See Figure 4 for scale of abundance, a. Fish eggs, b. Sprattus sprattus, c. Callionymus lyra, d. Gobiidae, e. Gymnammodytes semisquamatus, f. Microstomus kitt, g. Trisopterus spp., h. Pomatoschistus norvegicus.

75 modytes semisquamatus (11%). The gobies (Gobiidae) Distribution patterns for fish eggs and the dominant accounted for a further 2% of larval fish abundance and larval fish species, during June, are illustrated in Figure the remaining species, comprising several inshore types, 5. Patterns among the different larval species are very were sparse. similar. In general, levels of larval abundance were The patterns of distribution of the dominant ich­ higher at the outermost stations than in the shallow thyoplankton species, during April, are illustrated in waters of the bays. In addition, the larvae were recorded Figure 4. Most larvae were recorded at stations in inner predominantly at depths from 20 to 40 m in the water Bantry Bay and throughout Long Island Bay with very column. few occurring at intervening stations. Both sand-eel species (A. tobianus and G. semisquamatus) were re­ Patterns in distribution and abundance of corded predominantly in the Long Island Bay area (Fig. ichthyoplankton species in relation to the 4c, d). Although high levels of abundance of sprat eggs environment were recorded at all stations, levels were exceptionally high (up to 84 eggs m-3) in mid-Bantry Bay (Fig. 4a). April Highest levels of abundance of sprat larvae were also Numerical classification of the April data set produced recorded in Bantry Bay (Fig. 4b). three major groups of samples which were further sub­ Both diversity and abundance levels of larval fish divided into a total of seven sub-groups (Fig. 6a). This species were higher during June than in April (Table 2, sample grouping clearly reflected differences in station Fig. 5). A total of 29 species, plus Trisopterus spp. and locations (Fig. 1). With one exception, samples belong­ members of the goby family, were recorded on this ing to groups 1(a) and 1(b) were taken in the Long occasion and eight dominant taxa together accounted Island Bay area while those in 1(c) were from outer for 92% of total larval fish abundance. Sprat larvae Bantry Bay. Groups 2(a) and 2(b) consisted almost were most abundant, accounting for 60.2%, followed entirely of samples taken at the outermost stations, e.g. by Callionymus lyra, the gobies (Gobiidae), G ym ­ those along the transect joining Bantry and Fastnet. nammodytes semisquamatus, Microstomus kitt, Tris­ Samples belonging to groups 3(a) and 3(b) were taken opterus spp., Pomatoschistus norvegicus and Limanda exclusively at stations in inner Bantry Bay. limanda, each accounting for from 1.87 to 7.3% of total Figure 6b is the dendrogram which resulted from the abundance (Table 2). variable classification of the April data set. Five variable groups are identifiable. The most closely related vari­ ables belong to group 2, indicating that abundance of decapod larvae, fish eggs, Calanus spp., total copepods Table 3. Interpretation of variable codes in Figures 6b and 8b and Pseudocalanus elongatus, together with tem­ and in Tables 4 and 5. perature and dissolved oxygen, displayed similar pat­ Variable type Code Name terns of spatial variation. The variables depth, salinity, and cirripede larvae were also closely related in group Physical D Depth 1 and this group was linked to group 2 at a high level T Temperature of similarity on the dendrogram. Euphausiid eggs and S Salinity o Oxygen larvae were grouped with the copepod Metridia lucens Ichthyoplankton SP Sprattus sprattus in group 3 and the remaining three copepod species Taxa (Larvae) TR Trisopterus spp. were more closely related in group 4. Group 5 variables, GS Gymnammodytes semisquamatus consisting of total and individual species of fish larvae AT Ammodytes tobianus AM Ammodytidae (Family) and the chaetognaths Sagitta setosa and S. elegans, were GO Gobiidae (Family) weakly linked both to each other and to the former CL Callionymus lyra variable groups. TF Total fish larvae Results of nodal analysis, investigating relationships (Eggs) FE Fish eggs between sample and variable groups for the April data Zooplankton CO Total Copepoda Taxa CA Total Calanus spp. set, are presented in Table 4. It can be seen that most PE Pseudocalanus elongatus of the variables included in the analysis contributed TE Temora longicornis significantly to the sample grouping and that a high ML Metridia lucens degree of variation in mean levels of species abundance Centropages hamatus CH occurred between sample groups. Highest F-statistic AC Acartia clausii EE Euphausiid eggs values were calculated for salinity and for abundance EL Euphausiid larvae of the copepod species Pseudocalanus elongatus, Cen- DC Total Decapoda tropages hamatus, Acartia claussii and Temora longi- Cl Cirripede larvae cornis, total copepods, and euphausiid eggs. SE Sagitta elegans SS Sagitta setosa It is apparent that the major difference between the sample groups is manifest in the levels of abundance of

76 10 9 -

0 - 97 -

0-85

0-72

IH di O o 0 - 60 -

0 - 48 -

0 - 35-

0 - 23-

0 - 11 -

Depth (m) »nooooooooooißooooomißoioinißinoooooooogoooooooooooooooinoinooinoinoißoinoinT-T-T-cvicOrfCVlCOinCO r-*-r-C M «-.-*-C M »-lOCO’-fO C O ’-C'JCOCMCOCVJC’Jin ^ J-lß ’-CM fO’-C'it-COlO r- *- r- CM ^ CM CM CM r- y- Stn. No. ^CJCOT-CDCOCOCMCMCVi^COC»5l/)r*-S.^TjCOCO^, incO N .O ’-COCMCOS-0)0)0)OOCMCMOCOCM»-»-*-C,3COCOCOCO^'^lßmcDh-

Figure 6a. Sample grouping resulting from numerical classification of April data.

species, particularly of zooplankton, recorded (Table Bay, and the area in-between. In terms of the distri­ 4). For instance, sample group 3(a) is characterized bution of ichthyoplankton species, it can be seen that by exceptionally high levels of abundance of cirripede the occurrence of highest levels of abundance of sprat larvae, most species of copepods, fish eggs (sprat), and eggs and larvae coincided with the occurrence of highest sprat larvae. In contrast, the most notable feature of levels of abundance of several copepod species and the first major sample grouping (1) is the low levels of cirripede larvae in the reduced-salinity water in inner abundance of all copepod species, sprat eggs, and sprat Bantry Bay. In contrast, the two sand-eel species, A. larvae, whereas levels of abundance of the sand-eel tobianus and G. semisquamatus, were associated with larvae are at their highest. Furthermore, sample group high-salinity water, mainly in the Long Island Bay area, 2(b) is characterized by exceptionally high levels of in which levels of abundance of copepod species were abundance of euphausiids and the copepods Calanus low. spp. and M. lucens and by very low levels of abundance of fish eggs and larvae. June The distribution of the sample groups and their associ­ The dendrogram in Figure 8a shows the sample grouping ated plankton assemblages, throughout the sampling which resulted from the numerical classification of the area, is shown in Figure 7. There is a clear association June data set. Two main sample groups emerged and between sample groups 3(a) and 3(b) and the water of further subdivisions were identified to yield a total of reduced salinity (<34.70 ppt) which was confined in its five groups. In contrast to the April situation, the sample occurrence to inner Bantry Bay (see also Table 4 and grouping for June did not clearly reflect differences in Fig. 2). The remaining sample groups were associ­ station locations (Fig. 1). Sample groups 1(a) and 2(a) ated with the high-salinity water (>34.80 ppt) which included samples from all regions of the sampling area extended throughout Long Island Bay, outer Bantry and group 1(b) represented all regions except inner

77 1 73-

1 53-

c 0) •fi 1 33- u •H M-l « o 1-13- o

0 93-

0 73-

0 54-

0 34- 0 14- h -j -j X Variables : J lu U

Figure 6b. Variable grouping resulting from numerical classification of April data. (See Table 3 for interpretation of variable codes.)

Bantry Bay. Samples in group 2(b), however, were between highest levels of abundance of fish eggs and taken chiefly in Bantry Bay and the few in group 3 were larvae and all copepod species in sample groups 1(a) taken mainly in the Long Island Bay area. and 1(b), and the converse for sample group 3 in which Five groups of variables are discernible in the den­ levels of abundance of both copepod and ichthyo­ drogram yielded by the variable classification (Fig. 8b). plankton species were at a minimum. The most closely related variables belonged to groups 1, The distribution of the sample groups, and associated 2, and 3, which included the physical variables, decapod plankton assemblages, along the sampling transects is larvae, and all the copepod taxa. The remaining two illustrated in Figure 9. It can be seen that the assemblage groups, 4 and 5, consisted entirely of ichthyoplankton characterized by highest levels of abundance of ich­ taxa, except for the chaetognath S. elegans. In general, thyoplankton and copepods occurred in all areas except the ichthyoplankton taxa were weakly linked both to inner Bantry Bay. There was also a significant associ­ each other and to the other variables. ation between this assemblage and water of lowest Results of nodal analysis are given in Table 5. As for temperature which occurred predominantly below 20 m April, the copepod species, which were most abundant, depth (Table 5, Fig. 3). yielded highest F-statistic values, thus indicating their importance in the sample grouping. Unlike April, how­ ever, there was no significant difference in salinity values Discussion between the sample groups. Again, as for April, the major differences between the sample groups are the Previous records of ichthyoplankton in coastal waters levels of abundance of the plankton species. Import­ southwest of Ireland indicate that the peak spawning ant characteristics in this respect are the association period for the dominant spring-spawning species of fish

78 Table 4. Nodal analysis of April numerical classification results. Mean levels of abundance (no. m 3) of zooplankton and ichthyoplankton species and mean values for depth (m), temperature (°C), salinity (S) and dissolved oxygen (ml I ') in sample groups, and corresponding F-statistic values.

Variable group 1 2

Sample group DS Cl TO DC FECA CO PE

1 (a) 20.0 34.04 88.03 7.05 6.81 11.27 3.87 2.90 5.73 1.60 1 (b) 15.3 34.87 2.29 7.93 6.77 12.03 3.78 2.54 10.46 5.99 1(c) 42.5 34.97 1.32 8.14 6.76 0.04 1.91 0.89 3.54 0.96 2 (a) 27.9 34.95 24.66 8.09 6.86 4.50 7.00 2.98 9.95 3.32 2 (b) 24.0 .34.90 267.18 8.04 6.80 2.74 3.35 10.52 32.52 5.94 3(a) 10.0 34.62 1032.38 7.94 7.04 8.48 52.75 7.78 254.76 115.37 3(b) 15.8 34.54 9.90 7.91 6.90 3.70 3.49 3.51 69.99 27.27 * *** * * ** *** * * *** *** F-statistic 4.5 25.60 12.39 9.85 3.86 17.59 5.42 6.15 53.38 33.73

Variable group 3 4 5

Sample group EE EL MLCH AC TETF AT SP ST GS SE

1 (a) 0.11 0.17 0.42 0.07 0.12 0.11 0.58 0.51 0.02 0.02 0.02 1 (b) 0.32 0.22 0.86 0.32 0.05 0.32 0.59 0.37 0.05 0.12 0.06 1(c) 0.85 0.26 1.43 0.02 0.02 0.02 0.02 0.06 2(a) 5.60 1.15 0.99 0.12 0.17 0.15 0.03 0.01 0.01 0.04 2(b) 23.28 1.07 7.55 0.85 1.63 1.35 0.04 0.04 0.29 3(a) 1.03 0.68 97.63 15.75 8.13 0.58 0.02 0.41 0.35 0.06 0.01 3(b) 0.59 1.01 19.43 8.43 5.78 0.17 0.07 0.03 0.01 0.04 * * * * * * * *** *** * * * * F-statistic 47.38 6.53 2.94 98.02 50.04 38.1 8.89 9.95 5.07 3.86 3.84 2.79

See Table 3 for interpretation of variable codes. Significance levels (F-statistic): * - 0.05; ** - 0.01; *** - 0.001.

Sample-Group Symbols :

Group 1(a) - Group 2(a) Group 3(a) - ■ Group 1 (b) - Group 2(b) - i Group 3(b) - O Group 1(c) -

• •

20 -

40-

60 -

BANTRY BAY BANTRY BAY - FASTNET LONG ISLAND BAY

80 -

Figure 7. Distribution of sample groups along station transects during April.

79 in this area takes place during April and that peak levels advances. During April, levels of abundance of sprat of abundance of their larvae occur in the plankton eggs and larvae and sand-eel larvae were highest at the during May (Walshe, 1980; Grainger and Woodlock, shallow-water stations in Bantry and Long Island Bays. 1981; Doyle, 1988). The low diversity and levels of In contrast, during June, an increase in larval abundance abundance of larval fish species recorded during April of these and several other species was apparent with 1979, therefore, probably represent the situation in the progression into deeper water. Similar observations plankton of this region prior to the occurrence of the were made by Wallace and Pleasants (1972) and Grain­ annual peak in larval fish abundance. In contrast, the ger and Woodlock (1981) in relation to sprat spawning June records, comprising higher numbers and levels in the Celtic Sea area off the south coast of Ireland. of abundance of larval fish species, are likely to be They observed that spawning was restricted to the representative of conditions shortly after this annual coastal zone until April and subsequently it extended peak, when numbers of fish larvae in the plankton are offshore. beginning to decline. The extremely high numbers of This seaward progression in spawning activity of cer­ sprat eggs recorded during April are indicative of a peak tain fish species may possibly be related to water tem­ in spawning activity for this species. perature. Spawning activity of the spring-spawning Patterns in distribution of fish eggs and recently- species, in coastal waters west of Ireland, is most intense hatched larvae largely reflect the spatial spawning from February to April when temperatures are increas­ behaviour of the adult fish (Hewitt, 1981; Smith, 1981). ing gradually over the range 6-10°C, approximately Comparison between the distribution patterns of ich­ (Doyle, 1988). It is possible, therefore, that later in the thyoplankton species observed during April and June spawning season, when temperatures have increased suggests that a seaward progression in spawning activity, above 10°C, e.g. during May and June, the adult fish will of the fish concerned, may occur as the spawning season spawn preferentially in offshore, deeper water where

1-65-

1-48-

1-29-

1 - 10 -

0-91-

0-72-

0 53- 1(a) 1 (b) 2 (b) 0 34- 2(a)

0-15- iT l

o o o o o o O O O o o Depth (m) CM CM CM C3 C*3 Oy- On ifl *-O O•- CVJ T- CM S tu o No . r-o)cOi-t-o5fCvjco(Dco^in'Tj-in-«j-in'-cvjo>ocococMcocOfO'>t-«rOTrcMO)N-coocor-N-cM(r)>-Wh«cor)eoeO'«fCMinincocor^

Figure 8a. Sample grouping resulting from numerical classification of June data.

80 1- 59-

1-42-

1-24-

c

u •H U 0-71- 03 r—I •H S •H CO 0-53- CO

0-36-

0-18-

Variables

Figure 8b. Variable grouping resulting from numerical classification of June data. (See Table 3 for interpretation of variable codes.)

temperatures are lowest. This could explain the patterns highest levels of abundance of both were recorded at the of larval fish distribution observed in the study area deepwater stations. This hypothesis remains tentative, during June when larvae were recorded predominantly however, in that information concerning the temporal at the outermost stations and at the depths from 20 to and spatial patterns of primary and secondary pro­ 40 m in the water column, where temperatures ranged duction in the plankton of coastal waters southwest of from 9.5 to 10.5°C. Ireland is extremely limited. The offshore progression of spawning activity of fish The sample grouping resulting from numerical in coastal waters southwest of Ireland may also be classification of the first data set suggests that three related to patterns of production in the plankton. It is geographically distinct zooplankton assemblages existed known that off the west coast of Ireland the spring peaks in the sampling area during April and that their occur­ in phytoplankton and copepod production occur earlier rence was reflected in the distribution patterns of the in inshore and coastal waters than in the deep water of ichthyoplankton. Sprat eggs and larvae were associated the shelf and beyond (Colebrook and Robinson, 1965; with a coastal zooplankton assemblage which occurred Cronin, 1987; Doyle, 1988). It is possible, therefore, in water of reduced salinity in Bantry Bay and which that the seasonal, seaward progression of spawning may was characterized by very high levels of abundance be linked to a spatial variation in the commencement of of cirripede larvae and most copepod species and the production in the plankton of southwest coastal waters. occurrence of the chaetognath Sagitta setosa. The ex­ The spatial patterns in the plankton observed during tremely high levels of abundance of sprat eggs indicate this study seem to support such a hypothesis. Highest that spawning of this species was most intense in this levels of abundance of zooplankton and ichthyoplank­ type of water. Spawning behaviour may have been ton species occurred in the shallower areas of Bantry influenced significantly by the plankton content of the and Long Island Bays during April, whereas in June, water. The association of the sand-eel larvae (A. tobia-

81 Table 5. Nodal analysis of June numerical classification results. Mean levels of abundance (no. m '3) of zooplankton and ichthyoplankton species and mean values for depth (m), temperature (°C), salinity (S) and dissolved oxygen (ml L 1) in sample groups, and corresponding F-statistic values.

Variable group 1 2

Sample group DS T O DC CO CA AC

49.65 1 (a) 18.5 34.82 10.86 7.08 27.87 248.70 108.92 1 (b) 33.9 34.96 9.92 6.53 16.71 1024.42 88.75 18.86 2(a) 13.1 34.89 10.50 6.87 34.84 9.45 1.84 5.42 2(b) 17.8 34.80 10.63 7.02 20.64 40.62 18.02 12.14 3 7.1 34.91 11.30 7.39 49.53 1.00 0.14 0.30 ** * * * * * * * * ** *** F-statistic 7.9 1.13 4.80 5.56 4.19 240.38 18.69 22.00

Variable group 3 4 5

Sample group ML CH PE TE FE TF SP TR AMGO CL SE

1 (a) 20.43 28.06 33.88 5.78 3.08 2.12 1.26 0.12 0.14 0.14 0.21 0.35 1(b) 460.21 58.29 312.86 66.71 3.43 1.70 1.02 0.10 0.07 0.10 0.17 0.58 2(a) 0.20 0.29 0.42 0.73 1.21 1.16 0.76 0.01 0.13 0.07 0.05 0.01 2(b) 0.82 1.35 3.27 2.21 1.86 0.92 0.58 0.01 0.09 0.05 0.02 0.13 3 0.06 0.078 0.12 1.39 0.34 0.18 0.01 0.04 0.04 0.04 0.02 * * * *** **** *** ** * F-statistic 42.91 28.22 59.11 39.49 1.28 3.39 2.46 1.94 0.86 1.21 7.61 5.43

See Table 3 for interpretation of variable codes. Significance levels (F-statistic): * - 0.10; ** - 0.05; *** - 0.01; **** - 0.001.

nus and G. semisquamatus) with a second zooplankton butions. The salinity difference (in the region of 0.3 ppt) assemblage, characterized by low levels of abundance between the above two water types is considered too of all copepods and highest densities of decapod larvae, slight, in this instance, to have any influence on the which occurred in high-salinity water in the Long Island spawning behaviour of the fish or the subsequent larval Bay area, suggests that the distribution patterns of these behaviour and thereby the ichthyoplankton distri­ species may also be related to the zooplankton distri­ butions.

Sample-Group Symbols :

Group 1(a) - □ Group 2(a) - O Group 3 - a Group 1(b) - ■ Group 2(b) - •

m ■ □

20 - 20

60 - 60

BANTRY BAY BANTRY BÀY - PÄSTNET L?NÇ l$LAN,p £A,I

80 “

Figure 9. Distribution of sample groups along station transects during June.

82 In the deepwater region of the sampling area, a Alternatively, the unique patterns of distribution paucity of fish larvae was associated with a third observed for the sand-eel larvae may be related to the zooplankton assemblage characterized by the occur­ different spawning habits of the adult fish, i.e. sand-eels rence of neritic type zooplankton such as euphausiid are demersal spawners whereas the other fish species eggs and larvae, the copepods Calanus spp. and Metridia encountered are pelagic spawners. The substratum pref­ lucens and the chaetognath Sagitta elegans. It is clear, erence of the adult may therefore be the dominant however, that some spawning of sprat took place in this influential factor in relation to the distribution of young type of water as levels of sprat egg abundance were sand-eel larvae in the plankton. The species A. tobianus similar to those recorded in Long Island Bay. lives and spawns on clean, fine sand and G. semisqua­ Numerical classification results for June imply that matus prefers shell/gravel ground (Wheeler, 1969). The two main zooplankton assemblages existed in the area substratum throughout Bantry Bay consists of mud and and the ichthyoplankton distributions were related to sandy-mud, whereas the Long Island Bay area is char­ them. The first assemblage, characterized by excep­ acterized by a variety of substrata including fine sand tionally high levels of abundance of all copepod species, and shell, coarse gravel and shell and some mud (B. also contained highest levels of abundance of fish eggs O’Connor, pers. comm.). The association of sand-eel and larvae. It seems that spawning of the adult fish, larvae with the Long Island Bay area appears, therefore, including sprat, Trisopterus spp., Gobiidae and Cal­ to reflect the substratum preference of the adults. lionymus lyra, may have been associated with zones where high densities of copepods occurred. The second Acknowledgements assemblage was characterized by very low numbers of copepods plus lowest levels of occurrence and abun­ Thanks are due to Dr Brendan O’Connor, Marine dance of all the ichthyoplankton taxa, except for sand- Benthic Unit, Zoology Department, University Col­ eel larvae. lege, Galway, Ireland, for providing information (un­ The general implication from the above results is that published) on the nature of bottom sediments in the the observed relationships between ichthyoplankton study area. and zooplankton distribution patterns are functional and trophic in nature. Spatial variation in the planktonic References environment appears to be an influential factor in both Boesch, D. F. 1977. Application of numerical classification in spawning behaviour of the adult fish and larval behav­ ecological investigations of water pollution. Environmental iour. It seems that, initially, spatial spawning patterns Protection Agency Ecol. Res. Ser. EPA-600/3-77-033. 115 result in the association of patches of fish eggs and pp. recently-hatched larvae with patches of zooplankton Bray, J. R., and Curtis, J. T. 1957. An ordination of the upland forest communities of southern Wisconsin. Ecol. Monogr., organisms, particularly copepods whose suitability and 27: 325-349. importance as larval fish food is well documented. Sub­ Clarke, G. L., and Bumpus, D. F. 1950. The planktonsampler, sequently, behaviour of the larvae, such as vertical an instrument for quantitative plankton investigations. Spec. migration or some sort of aggregating mechanism, may Pub. Am. Soc. Limnol. Oceanog., No. 5. 8 pp. Clifford, H. T., and Stephenson, W. 1975. An introduction to serve to maintain the larvae in regions of high prey numerical classification. Academic Press, New York. 229 density. It is known, for instance, that larvae of the pp. northern anchovy Engraulis mordax have the sensory Colebrook, J. M., and Robinson, G. A. 1965. Continuous and motor capacity to locate and remain in dense plankton records: seasonal cycles of phytoplankton and patches of food organisms (Smith and Lasker, 1978). copepods in the north-eastern Atlantic and the North Sea. Bull. Mar. Ecol., 6: 123-139. Such behaviour, of both adults and larvae, and the Cronin, A. 1987. A study of the phytoplankton ecology of resulting spatial agreement between the occurrence of the Inner Galway Bay. PhD thesis. National University of fish larvae and their food organisms, are obviously of Ireland. adaptive significance in that they are likely to enhance Doyle, M. 1988. An ecological study of fish larvae in the plankton of coastal waters west of Ireland. PhD thesis. the survival and development of larvae. National University of Ireland. The distinct lack of association between the distri­ Ellertsen, B., Fossum, P., Solemdal, P., Sundby, S., and bution of sand-eel larvae and copepods, observed during Tilseth, S. 1984. A case study on the distribution of cod the present study, seems unusual. Other workers have larvae and availability of prey organisms in relation to physi­ found that sand-eel larvae (Ammodytes marinus in this cal processes in Lofoten. In The propagation of cod (Gadus morhua L.), pp, 453-457. Ed. by E. Dahl, D. S. Danielssen, instance) were more specialized in their feeding habits, E. Moksness, and P. Solemdal. Flødevigen Rapportser 1. compared with other larval fish species (Ryland, 1964; Frank, K. T., and Leggett, W. C. 1983. Multispecies larval Wyatt, 1974). It may be the case, therefore, that plank­ fish associations: accident or adaptation. Can. J. Fish. ton organisms other than copepods are most important Aquat. Sei., 40: 754-762. Gauch, H. G. 1982. Multivariate analysis in community ecol­ in the diet of sand-eel larvae in Irish coastal waters. ogy. Cambridge University Press, Cambridge. 298 pp. Such a hypothesis, however, needs further inves­ Grainger, R. G. R., and Woodlock, J. 1981. Sprat egg surveys tigation. off the south coast of Ireland in 1980. ICES CM 1981/B: 43.

83 Hewitt, R. 1981. The value of pattern in the distribution of in the southern North Sea. J. Mar. Biol. Ass. U.K., 44: 343- young fish. Rapp. P.-v. Réun. Cons. Perm. int. Explor. 364. Mer, 178: 229-236. Smith, P. E. 1981. Larval anchovy patchiness. In Marine fish Hjort, J. 1914. Fluctuations in the great sea fisheries of north­ larvae: morphology, ecology and relation to fisheries, pp. ern Europe viewed in the light of biological research. Rapp. 22-31. Ed. by R. Lasker. University of Washington Press, P.-v. Réun. Cons. int. Explor. Mer, 20: 1-228. Seattle. 131 pp. Hunter, J. R. 1981. Feeding ecology and predation of marine Smith, P. E., and Lasker, R. 1978. Position of larval fish in an fish larvae. In Marine fish larvae: morphology, ecology and ecosystem. Rapp. P.-v. Réun. Cons. int. Explor. Mer, 173: relation to fisheries, pp. 33-77. Ed. by R. Lasker. University 77-84. of Washington Press, Seattle. 131 pp. Strickland, J. D., and Parsons, T. R. 1972. A practical hand­ Lance, G. N., and Williams, W. T. 1967. A general theory book of seawater analysis. Bull. Fish. Res. Bd. Can., 169: of classificatory sorting strategies. I. Hierarchical systems. 311 pp. Computer J., 9: 373-380. Wallace, P. D., and Pleasants, C. A. 1972. The distribution of Lasker, R. 1981. The role of a stable ocean in larval fish eggs and larvae of some pelagic fish species in the English survival and subsequent recruitment. In Marine fish larvae: Channel and adjacent waters in 1967 and 1968. ICES CM morphology, ecology and relation to fisheries, pp. 80-87. 1972/J: 8. Ed. by R. Lasker. University of Washington Press, Seattle, Walshe, B. G. 1980. A survey of the ichthyoplankton of the 131 pp. south coast of Ireland and Bantry Bay. BSc thesis. Zoology Lasker, R. 1985. What limits Clupeoid production? Can. J. Department, University College, Galway, Ireland. Fish. Aquat. Sei., 42: 31-38. Wheeler, A. 1969. The fishes of the British Isles and north­ Paulsen, H. 1985. Effects of food patches on distribution, west Europe. Macmillan, London. 613 pp. growth and survival of fish larvae - a summary of exper­ Wishart, D. 1978. CLUSTAN users manual. Edinburgh Uni­ imental results. ICES CM 1985/L: 22. versity, Edinburgh. 175 pp. Russell, F. S. 1976. The eggs and planktonic stages of British Wyatt, T. 1974. The feeding of plaice and sand-eel larvae in marine fishes. Academic Press, London. 524 pp. the Southern Bight in relation to the distribution of their Ryan, T. A. 1982. Zooplankton, temperature and salinity in food organisms. In The early life history of fish, pp. 245- Bantry Bay and Long Island Bay, southwest coast of Ireland. 251. Ed. by J. H. S. Blaxter. Proc. Int. Symp., Springer- MSc thesis. National University of Ireland. Verlag, Berlin. 765 pp. Ryland, J. S. 1964. The feeding of plaice and sand-eel larvae

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