BULLETIN OF MARINE SCIENCE, 71(1): 97–108, 2002
DISTRIBUTION OF JUVENILE SQUID IN THE SCOTIA SEA IN RELATION TO REGIONAL OCEANOGRAPHY
C. I. H. Anderson and P. G. Rodhouse
ABSTRACT A total of 211 juvenile and paralarval squid were collected on five research cruises made in the Scotia Sea region during the austral summer (1996–1999). These included specimens of Alluroteuthis antarcticus (Neoteuthidae), Batoteuthis skolops (Batoteuthidae), Brachioteuthis sp. (Brachioteuthidae), Galiteuthis glacialis (Cranchiidae), Gonatus antarcticus (Gonatidae), Mesonychoteuthis hamiltoni (Cranchiidae), Psychroteuthis glacialis (Psychroteuthidae), and a number of small onychoteuthids. The specimens ranged in size from 3.8 to 51.9 mm (dorsal mantle length), and significant differences in the size of the specimens collected were found both between and within species. Water mass type, ocean depth, daylight state and cruise (year) all had significant effects on the overall pattern of catches of squid per trawl under specific circumstances, but no significant differences were found in the pattern of catches of the individual spe- cies. Indications were found that different species favour different water masses, that the near shelf environment may be the most productive for catches of juvenile squid, and that there are interannual differences in the catches of juvenile squid in the vicinity of South Georgia. Overall, although based on a small sample of specimens, this study found that the regional oceanography does influence the distribution of juvenile squid in the Scotia Sea.
In the Scotia Sea region, relatively little work has been done on the distribution of paralarval and small juvenile squid (sensu Young and Harman, 1988), and what studies there are have generally been restricted in either temporal or spatial extent (e.g., Rodhouse, 1989; Rodhouse and Piatkowski, 1995). While most work has focussed on the vertical distribution of species within the water column (Rodhouse and Clarke, 1985; Rodhouse and Clarke, 1986; Piatkowski et al., 1994; Rodhouse and Piatkowski, 1995), no multispecies studies from anywhere in the Antarctic have looked at relationships with specific water masses. However, with the increasing emphasis on the ecosystem approach to understanding marine systems (e.g., Murphy et al., 1998), and the recognition of the influence of the physical environment on squid recruitment dynamics (e.g., Waluda et al., 1999), it is clear that investigating these relationships may yield new insights into the ecology of the early life history stages of Antarctic squid. The surface waters of the Scotia Sea can be separated by region on the basis of water masses, each having distinctive temperature-salinity characteristics. These water masses are not static features. They vary in nature seasonally and are advected around Antarctica in the flow of the Antarctic Circumpolar Current (ACC) (Nowlin and Klinck, 1986). Their margins are defined by clear oceanic fronts associated with high velocity current jets, and may develop distinct frontal features such as eddies and meanders (Bryden, 1983). Because of these characteristics, both the water masses themselves and their bound- aries are associated with specific biotic and abiotic conditions which influence the vari- ety and abundance of species found within them (Longhurst, 1998). A clear example is provided by the Polar Front which separates Sub-Antarctic Surface Water from Antarctic
97 98 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002
Zone Water (Peterson and Whitworth, 1989), and is associated with an area of seasonally enhanced primary productivity (Longhurst, 1998). This study describes the distribution of paralarval and small juvenile squid collected over a series of cruises in the Scotia Sea during the austral summer, and draws inferences on their relationship to the regional oceanography and other environmental variables, such as water depth and the diurnal cycle.
MATERIALS AND METHODS
Juvenile squid were collected on five cruises of the British Antarctic Survey research vessel JAMES CLARK ROSS between 1996 and 1999 (Table1). Four of those cruises (JR11, JR17, JR28 and JR38) comprised the Pelagic Ecosystem Studies Group ‘Core Programme’, studying interannual variability in the marine ecosystem immediately to the north of South Georgia. The fifth cruise, the ‘GeneFlow’ cruise (JR26), circumnavigated the Scotia Sea to collect samples for genetic analysis from across the region. A similar methodology was used on all five cruises (see North et al., 1998) and a complete suite of oceanographic data, including temperature and salinity profiles, was col- lected in parallel with the biological sampling. A variety of nets were used to collect specimens, with the majority of successful hauls made by a standard rectangular midwater trawl with an 8 m2 aperture (RMT8), and a neuston net (FNET) (1 m2 aperture) deployed from the foredeck. The only significant difference in methodology between the cruises was that during the ‘Core Programme’ cruises standardized RMT8 net hauls and those targeted at acoustic marks were carried out, whilst on the ‘GeneFlow’ cruise only the latter were used. The standardized net hauls were fished along oblique profiles between the surface and approximately 200 m depth over 30 min periods, while the target net hauls followed a variety of profiles at a range of depths between the surface and 600 m. All the juvenile squid collected were preserved (predominantly in 95–99% ethanol) upon collec- tion, and identified at a later date (using Fischer and Hureau, 1985; Sweeney et al., 1992). Identifi- cations were made to species level wherever possible, and at least to family level in most cases. The dorsal mantle length (ML) of all intact specimens was measured (to an accuracy of approximately ± 0.1 mm) on identification. Only specimens preserved exclusively in ethanol were used in the analysis of mantle lengths. All statistical analyses were performed using Minitab ver. 12.22 (Minitab Inc., 1998), except the G tests (Sokal and Rohlf (1995) with recommended Williams correction (p. 698)) which were calculated by hand (critical values from Rohlf and Sokal, 1995). The mean ML of the specimens caught was compared between the species, using the specimens from all the cruises pooled. For the most abundant species (Brachitoteuthis sp. and Galiteuthis glacialis), the mean ML was also com- pared between cruises for each species separately. The effects of environmental variables on squid catches were investigated using the frequency distributions of (1) total catches of squid in each haul and (2) total catches of each species in each haul. Statistical tests were made examining the effects of water mass type (‘GeneFlow’ data only), daylight state, water depth and, for the ‘Core Programme’ data, cruise (year) and location within the sampling area (see Table 2 for details).
RESULTS
A total of 211 squid were collected of which 197 could be identified to family or spe- cies level (Table 1). Among the specimens not identifiable to the species level was a neoteuthid (ML 28.7 mm) collected near the Burdwood Bank to the south of the Falkland Islands (at 54.40°S, 56.43°W). This shows some characteristics of Neoteuthis spp. (Sweeney et al., 1992), with a long narrow fin shape and a fin length of 67% ML, but the morphology of the club is closer to that described for Nototeuthis dimegacotyle Nesis and Nikitina 1986 with two greatly enlarged suckers on the manus. As the type specimen of ANDERSON AND RODHOUSE: JUVENILE SQUID DISTRIBUTION IN THE SCOTIA SEA 99
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* 100 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002
Table 2. Categories used in the statistical analysis of the influence of environmental factors (with definitions).
Fsactor Categorie Dyaylight state Dta Ttwiligh Nigh Nautical dawn + 1hr Nautical dawn to Nautical dusk to to nautical dusk − 1hr nautical dawn + 1h; nautical dawn nautical dusk − 1hr to nautical dusk
Wfater depth Sehel Snhelf slop Ocea <200 m deep 200−1000 m deep >1000 m deep
Location (aSouth Georgia region) Eaastern are Wrestern are Othe 37.5ºW−4W0ºW 34.5º −36.25ºW
N. dimegacotyle was collected from an equivalent latitude off the Pacific coast of Chile (at 50°S, 81°W), it seems highly plausible that the current specimen is from the same species. 96.2% of the squid collected (203 individuals) were caught using the RMT8 net, so all further analyses are confined to the catches made with this net. Sampling was concen- trated in the top 150 m during the ‘GeneFlow’ cruise and was almost exclusively carried out in the top 300 m during the ‘Core Programme’ cruises. All species where more than five specimens were collected (Brachioteuthis sp., Gonatus antarcticus, Psychroteuthis glacialis, G. glacialis, and the onychoteuthids) were caught across the entire depth range sampled (surface to 600 m), and the wide range of fishing profiles used precluded an analysis of the vertical distribution of even the most abundant species. SIZE OF SQUID.—The size of the squid caught in the RMT8 net ranged from 3.8 mm ML (a probable onychoteuthid) to 51.9 mm ML (Brachioteuthis sp.), with a mean ML of 15.7 ± 9.4 (STD) mm (Table 1). The smallest individual caught on its own had a ML of 5.2 mm (Mesonychoteuthis hamiltoni). The size of the squid caught varied significantly between the species (ANOVA F = 66.36, df = 171, P < 0.001), with two distinct clusters of species seen (Table 1). The larger sized group (Mean ML > 25 mm) comprised Brachioteuthis sp and Batoteuthis skolops, and the smaller sized group (Mean ML < 20 mm) comprised G. glacialis, M. hamiltoni, G. antarcticus, P. glacialis and the onychoteuthids. Within the smaller sized group, G. antarcticus had a significantly higher mean ML than either G. glacialis or the onychoteuthids (Tukey’s pairwise comparisons, p < 0.05). Within spe- cies, significant differences in the size of specimens caught in each cruise were found for both Brachioteuthis sp. (ANOVA F = 3.74, df = 45, P = 0.011) and G. glacialis (ANOVA F = 6.10, df = 66, P < 0.001) (Table 3). For Brachioteuthis sp., the squid caught on JR26 (‘GeneFlow’) were significantly smaller than those caught on JR28 (‘Core Programme’ 3), whilst for G. glacialis the squid caught on JR17 (‘Core Programme’ 2) were signifi- cantly smaller than those caught on any other cruise (Tukey’s pairwise comparisons, p < 0.05). DAYLIGHT STATE AND OCEAN DEPTH.—During the ‘GeneFlow’ cruise, daylight state had no significant effect on the frequency distribution of total catches of squid, but ocean depth did (G Test: Corr. G = 20.39, df = 2, P < 0.001). Given the frequency distribution of the net hauls, far more squid than expected by chance were caught on the shelf slope, and to some extent in the ocean, and far fewer than expected were caught on the shelf. ANDERSON AND RODHOUSE: JUVENILE SQUID DISTRIBUTION IN THE SCOTIA SEA 101
Table 3. The mean size (ML ± STD) (mm) of the specimens of Galiteuthis glacialis and Brachioteithis sp. collected on each cruise using the RMT8 net.
Cruise Gs. glacialis Brachioteuthi sp. J5R11 11. ± 12.9 25. ± 9.9 (n = 4n) ( = 5) J9R17 7. ± 26.7 27. ± 10.0 (n = 1n0) ( = 7) J3R28 12. ± 13.6 33. ± 4.2 (n = 7n) ( = 10) J8R38 10. ± 19.9 33. ± 4.5 (n = 4n4) ( = 6) J8R26 12. ± 21.7 24. ± 7.4 (n = 2n) ( = 18)
During the ‘Core Programme’ cruises, both daylight state (G Test: Corr. G = 7.45, df = 2, P < 0.025) and water depth (G Test: Corr. G = 22.34, df = 2, P < 0.001) had a significant effect on the frequency distribution of total catches of squid. More squid than expected were caught during the day and fewer squid were caught at night, and far more squid than expected were caught in the ocean, compared to the shelf and shelf slope. WATER MASS TYPE (‘GENEFLOW’ CRUISE DATA).—During the ‘GeneFlow’ cruise, six different water masses were sampled with 65 RMT8 net hauls, and a total of 87 squid were caught in 20 of those hauls (Fig. 1). Squid were caught in Antarctic Surface Water (ASW), Sub-Antarctic Zone Water (SAZW) and Weddell-Scotia Confluence Water (WSC), but not in Western Antarctic Peninsula Water (WAPW), Weddell Sea Water (WSW) or
Figure 1. A map of the Scotia Sea region showing the location of all RMT8 net hauls made on the ‘GeneFlow’ cruise and the water masses in which they occurred (see text for details). The presence or absence of juvenile squid in the hauls is indicated, and the 200 m and 1000 m depth contours are shown. 102 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002
Figure 2. ‘GeneFlow’ Cruise Data: Graphs showing a) the number of RMT8 hauls made in each water mass (see text for details), and b) the number of individuals of each species caught in Antarctic Surface Water (ASW) and Sub-Antarctic Zone Water (SAZW). ANDERSON AND RODHOUSE: JUVENILE SQUID DISTRIBUTION IN THE SCOTIA SEA 103 mixed ASW/SAZW (Fig. 2A). The ASW, SAZW and WAPW were sampled with be- tween 11 and 28 RMT8 hauls, whilst the other water masses were only sampled by one RMT8 haul each. The frequency distribution of total catches of squid was significantly different between ASW, WAPW and SAZW (Kruskal-Wallis Test: Adj. H = 20.65, df = 2, P < 0.000), and the median catch of squid was significantly lower in ASW than SAZW (Mann-Whitney Test: W = 489.0, Adj. P = 0.009). Although clear differences can be seen in the distribu- tion of Brachioteuthis sp., G. antarcticus and the onychoteuthidae between ASW and SAZW(Fig. 2B), the frequency distribution of catches of individual species was not sig- nificantly different (Wilcoxon’s Signed-Ranks Test). INTERANNUAL VARIABILITY AT SOUTH GEORGIA (‘CORE PROGRAMME’ CRUISE DATA).—116 squid were collected in 40 out of a total of 98 RMT8 nets deployed during the four cruises (Fig. 3). Although the majority of net hauls occurred within two distinct areas (35.7% in the eastern area and 55.1% in the western area), location within the study area had no significant effect on the pattern of catches of all squid together, or on that of catches of Brachioteuthis sp. (Mann-Whitney Tests). It did have an effect on catches of G. glacialis, with significantly higher numbers caught in the western area than the eastern area (Mann- Whitney Test: W = 1383.0, Adj. P = 0.018). However, as such small numbers of speci- mens were available, data from across the South Georgia study area were pooled for all further analyses. The frequency distribution of catches of all squid together was significantly different between the different cruises (years) (Kruskal-Wallis Test: Adj. H = 12.61, df = 3, P = 0.006) (Fig. 4A). No test could be made of the influence of ‘cruise’ on the frequency distributions of the catches of the different species of squid together (Fig. 4B). However, it is clear that the main difference between the cruises is the extremely large increase in the number of G. glacialis caught during JR38, and that two other species are only caught during this cruise. Separate tests of the influence of ‘cruise’ were made for each of the
Figure 3. A map of the South Georgia region showing the location of all RMT8 net hauls made on the four ‘Core Programme’ cruises (JR11, JR17, JR28 and JR38). The presence or absence of juvenile squid in the hauls is indicated, and the 200 m and 1000 m depth contours are shown. 104 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002
Figure 4. ‘Core Programme’ Cruise Data: Graphs showing a) the number of RMT8 hauls made during each cruise, and b) the number of individuals of each species caught during each cruise. ANDERSON AND RODHOUSE: JUVENILE SQUID DISTRIBUTION IN THE SCOTIA SEA 105 three most abundant species Brachioteuthis sp., P. glacialis and G. glacialis. Neither Brachioteuthis sp. or P. glacialis showed significant between cruise differences in their frequency distribution of catches, unlike G. glacialis which did (Kruskal-Wallis Test: Adj. H = 18.92, df = 3, P < 0.001). Additionally, G. glacialis showed no significant effect of location (eastern or western area) during JR38 (Mann-Whitney Test).
DISCUSSION
During the following discussion, it should be remembered that the sampling techniques used in this study are not specifically designed to collect small cephalopods, but rather to sample a wide range of macro-zooplankton and micro-nekton. Small specimens, at the bottom of the size range collected, are likely to be under represented in this study. This is partly due to the size selectivity of the nets used, and partly due to the crudeness of the initial sorting of the net samples which will favour finding the smallest specimens only when other more visible squid are also present in the sample. Above a minimum size (approximately 6 mm ML), the sampling techniques used appear to have provided an adequate representation of the juvenile squid fauna present, although it should be remem- bered that not all species known from the region are found within this collection (Fischer and Hureau, 1985; Rodhouse, 1990). When looking at the size of the specimens caught, there are clear differences between the mean sizes of some of the species collected. This is likely to at least partly reflect the differing availability of these squid for capture by the net. The availability of squid maybe influenced both by differences in the rate of ontogenic descent, and so presence in the depth range sampled, and differences in the degree to which squid of a particular size may actively avoid the net. In the case of Brachioteuthis sp. (‘large size group’), this species is commonly caught at a wide range of sizes in the top 200 m of the water column (e.g., Lu and Williams, 1994; Piatkowski et al., 1994) and occasional adult specimens (ML >100 mm) have been caught within this depth strata using an RMT8 (Rodhouse et al., 1992). Together, these facts suggest both a persistence in surface waters and a reduced ability to escape the net even at large sizes for this species. In comparison, G. glacialis (‘small size group’) shows a well documented ontogenic descent to depths beyond those sampled in this study (Rodhouse and Clarke, 1986; Piatkowski and Hagen, 1994). It would therefore be premature to attempt to interpret the interspecific variation seen in the mean size of the different species more fully at this time. The differences in the mean ML of the specimens of Brachioteuthis sp. caught on different cruises can be primarily interpreted as caused by the range of times over which the cruises took place, with a larger average size seen later in the summer. However, the only significant differences seen were between the specimens from the earliest and latest occurring cruises (JR26 and JR28 respectively). In contrast, the variation in the size of G. glacialis suggests that there may also be a degree of interannual variability in the mean size of the squid caught, as the specimens from JR17 are significantly smaller than those from both JR28 (which covered the same period), and JR26 (which occurred earlier in the season). The varying effects of daylight state and ocean depth on the pattern of catches from the ‘GeneFlow’ and ‘Core Programme’ cruises are also revealing. That daylight state only has a significant effect on the ‘Core Programme’ data, and that more catches are made during the day than expected, is probably an artifact of the distribution of net hauls of 106 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002 different types. During the ‘Core Programme’ cruises, standard hauls are made exclu- sively at night and the majority of targeted hauls are made during the day. That no effect is seen in the ‘GeneFlow’ cruise data implies that either the squid caught are not making significant diurnal migrations or that the majority of the migrations are occurring within the depth range sampled. In both the ‘GeneFlow’ and ‘Core Programme’ data, fewer catches than expected are made on the ‘shelf’ (ie in water less than 200 m deep). It should be recognised that around the Antarctic Peninsula, where the shelf is depressed by the weight of the Antarctic ice cap, the 200 m isobath does not correspond to the physical shelf break, but it still acts as a proxy separating shallow coastal waters from deeper more oceanic-like areas. The pat- tern seen may be further biassed in the ‘GeneFlow’ data by the prevalence of these ‘shelf’ stations occurring in the Western Antarctic Peninsula Water where no squid were caught, but the general trend is confirmed by the similar pattern in the ‘Core Programme’ data. Comparison of the other effects of ocean depth on the two data sets, and the geographic distribution of the catches, suggests that the ‘shelf slope’ (from ‘GeneFlow’ data) and ‘ocean’ near the shelf slope (from ‘Core Programme’ data) may be the most productive environments for juvenile squid catches. This is in agreement with the results of previous studies from different parts of the Antarctic (Rodhouse, 1989; Lu and Williams, 1994), and may be related to the high concentrations of marine productivity often found in these areas (Longhurst, 1998). The distribution of squid was not uniform between the water masses sampled, and there are indications that different species favour different water masses. Among the more abundant species, Brachioteuthis sp. and P. glacialis were found exclusively to the south of the Polar Front, and G. antarcticus and the onychoteuthids were found predominantly to the north of it (in Sub-Antarctic Zone Water). That no squid were caught in the Western Antarctic Peninsula Water (WAPW) may be due to the characteristics of that water mass, to the dominance of shelf sampling stations in it, or to some seasonal effect on the avail- ability of juvenile cephalopods in the southern half of the Scotia Sea region. Previous cruises sampling the same part of the WAPW water mass have successfully caught juve- nile cephalopods using equivalent techniques (Rodhouse, 1989). However, the majority of these catches appear to have been made during the austral winter period (May–June), indicating that a temporal shift in the availability of juvenile cephalopods may be occur- ring in this area. The ‘Core Programme’ data show a significant degree of interannual variability in the pattern of catches made. This appears to be driven by interannual variability in the abun- dance of individual species, which then influences the size of the total catches made. Specifically, the catches made during JR38 were unusually high, and this can be attrib- uted to the unusually high numbers of G. glacialis caught. Given the apparent sensitivity of young squid to environmental conditions effecting growth and survival (Forsythe, 1993; Bakun and Csirke, 1998), this is ultimately likely to be related to oceanographic variabil- ity. The influence of oceanographic conditions on variability in other components of the South Georgia marine ecosystem has already been demonstrated (Murphy et al., 1998), but as no relevant analysis of oceanographic data from this study has been carried out to date, no firm conclusions can be drawn as to the causes of the variability in the squid described here. However, the indications of interannual variability seen in this study do highlight the need for repeated sampling over extended periods if a comprehensive pic- ture of the juvenile cephalopod fauna of an area is to be developed. ANDERSON AND RODHOUSE: JUVENILE SQUID DISTRIBUTION IN THE SCOTIA SEA 107
Overall, it can be seen that the regional oceanography and the bathymetric environ- ment of the Scotia Sea do appear to influence the distribution of juvenile squid. That significant relationships have been found, despite the low catch numbers, vindicates this approach as a useful method for expanding our understanding of the ecology of young squid.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the assistance of M. Brandon in providing the identification of water masses for ‘GeneFlow’ and general oceanographic advice, and of the masters, officers and crew of the JAMES CLARK ROSS for assistance in the field. I. Staniland and C. Waluda made many helpful comments during the drafting of this manuscript. C. I. H. A. was supported throughout this study by a NERC CASE Award.
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ADDRESSES: (C.I.H.A.) Department of Zoology, University of Aberdeen, Tillydrone Avenue, Aberdeen, AB24 2TZ, UK and British Antarctic Survey, High Cross, Madingley Road, Cambridge, CB3 0ET, UK. E-mail: