<I>Pagellus Bogaraveo</I>

BULLETIN OF MARINE SCIENCE, 69(3): 1073–1087, 2001

FEEDING HABITS OF TWO CONGENER SPECIES OF SEABREAMS, PAGELLUS BOGARAVEO AND PAGELLUS ACARNE, OFF THE AZORES (NORTHEASTERN ATLANTIC) DURING SPRING OF 1996 AND 1997

Telmo Morato, Encarnacion Solà, Maria Pitta Grós and Gui Menezes

ABSTRACT The blackspot seabream, Pagellus bogaraveo, has traditionally been the main target species of the bottom longline fishery in the Azores. However, in recent years several other species have increased in economic importance, including the axillary seabream, Pagellus acarne. Despite their commercial value, few studies regarding their diet composition were found in the literature. Data were collected during the demersal cruise surveys that took place aboard the RV ARQUIPÉLAGO during the spring of 1996 and 1997. The stomach contents of blackspot and axillary seabreams were examined to define their diets. Both seabreams studied fed on a wide variety of small organisms, mainly fishes and several invertebrates. Prey fishes were important in the diets of both blackspot seabream and axillary seabream (61.3 and 76.3% frequency of occurrence, respectively). Thaliaceans and ophiuroids were the most important invertebrates for both species. In addition, blackspot seabream also fed upon pelagic gastropods while axillary seabream consumed bivalves and echinoids. Prey composition of the blackspot seabream diet varied mainly due to the use of different environments, and consequently prey availability. The feeding behavior of this species appears to be size related, as small individuals are mostly males and live preferentially in coastal areas and shallower waters, and exhibit different feeding habits from those observed for larger individuals, which are mainly females and live preferentially at offshore banks and deeper waters. Prey composition indicated that both predators have the ability to feed near the bottom on benthic prey as well as pelagic species in the water column. This study suggests that in addition to locally produced food, the productivity of many seamounts communities is also dependent on a regular supply of productivity that drift past seamount. Although the results of the stomach contents analysis showed a significant feeding overlap, we suggest that they may not be sharing resources with each other, because the trophic activities of the two species are segregated by differences in habitat use. In this study, habitat partitioning appeared to be an important factor in preventing diet overlap.

In the Azores, the longline constitutes an important fishery and is directed towards demersal and deep-water species. This fishery experienced a rapid development in the 1980s, but the catch rates of the commercial fleet have declined in the last few years (Menezes et al., 1998). The blackspot seabream, Pagellus bogaraveo (Brünnich, 1768), has traditionally been the main target species of the bottom longline fishery in the Azores, but in recent years several other species have increased in economic importance, including the axillary seabream, Pagellus acarne (Risso, 1827). From 1981 to 1994 blackspot seabream represented 26% of total landings and 38% of total income of the demersal fisheries in the Azores. During this period, landings of this species increased by about 240% while an overall decrease in relative abundance was observed in the Azores (Menezes

1073 1074 BULLETIN OF MARINE SCIENCE, VOL. 69, NO. 3, 2001

and Silva, 1999). Silva et al. (1998) suggested the need to reduce the fishing effort in order to decrease the fishing mortality. Blackspot seabream displays an extensive vertical distribution, ranging from 0 to more than 600 m depth, usually above rocky or sandy bottoms (Menezes et al., 1998). Ontoge- netic changes of habitat are characteristic of this species, where juveniles appear in shallow coastal waters and adults in deeper waters at the offshore banks (unpubl. data). The blackspot seabream is a protandric hermaphrodite with sex inversion occurring between 27 and 34 cm FL (Krug, 1990). Axillary seabream, which is also a protandric hermaphrodite, is usually found above sandy bottoms between 0 and 350 m depth. In the Azores, the blackspot seabream spawns from January to April (Estácio et al., 2001) while the axillary seabream spawns from April to July (unpubl. data). Because of these biological charac- teristics, both species have been considered very vulnerable to exploitation. Some biological aspects of the blackspot seabream in the Azores have been extensively studied in the last few years. The age and growth have been described by several authors (see Krug, 1989, 1998; Menezes et al., 1998). Information on reproduction, including reproductive cycle, spawning period, maturity and fecundity, is also available (Krug, 1989; Estácio et al., 2001). More recently, some genetic studies have been carried out on blackspot seabream, including the isolation of ten microsatellite loci preformed by Stockey et al. (2000). The biology of the axillary seabream is presently being studied in the Azores. While other aspects of the life history are well described, the trophic ecology of these species is poorly known. The feeding habits of demersal fish species found in the Azores have been under investigation since 1996. Morato et al. (1999) presented data for the forkbeard, Phycis phycis, and the conger eel, Conger conger. However, data on diet and the feeding habits of both seabream species have not yet been published from the Azores. In general, the feeding habits and diet composition of these species have been poorly studied. Olaso and Pereda (1986) described the diet of 35 fish species in the Cantabrian Sea, including the blackspot seabream. The diet of the axillary seabream in the Mediter- ranean was described by Andoloro (1982, 1983), in the central eastern Atlantic by Domanevskaya and Patokina (1984), and in the Cantabrian Sea by Olaso and Rodríguez- Marín (1995). These studies only briefly described the diet of both species and did not examine any resource partitioning between them. We examined the diet of blackspot and axillary seabreams caught during the 1996 and 1997 demersal survey cruises, in order to describe the overall diets of these species, and to compare it (when sample sizes allowed) by fish size, sex, and depth, as well as area of capture.

METHODS

DATA COLLECTION.—A total of 1263 blackspot seabreams and 235 axillary seabreams were caught during a wider research program aimed at the study of the Azorean demersal fisheries using longline onboard the RV ARQUIPÉLAGO. Fishes were caught during the 1996 and 1997 demersal survey cruises that took place between March and May and covered coastal areas and offshore banks across the Azores (Fig. 1). The longline used in the experiments is identical to the one widely used in the commercial fisheries in the Azores. The standardized gear is locally known as stone-buoy longline and were performed with ca 128 hooks (MUSTAD 2335, no 9) per skate mounted on 40 cm branch- lines attached to the mainline at ca 1 m intervals and baited with pieces of salted sardine. Between each 32 hooks a small buoy or a stone were alternated attached to the mainline. Each fishing experi- MORATO ET AL.: FEEDING HABITS OF TWO CONGENER SPECIES OF SEABREAMS 1075

Figure 1. Sampling area and locations of the longline sets made in the Azores during the spring of 1996 and 1997. ment was conducted with about 40 skates per set, depending on bottom irregularity. Line setting began before sunrise (approx. 05:00) and hauling started after about 2 h. From the fish sampled, fork length (FL, to the lowest cm) was measured, and sex and maturity were determined macroscopically. Stomachs were classified as everted (E), regurgitated (R), with bait (B), empty (0), one quarter-full (1), half-full (2), three quarter-full (3) or full (4). Stomachs classified as category E, R or B, as well as individuals that had obviously eaten hooked fish from the longline, were excluded from further analysis. Stomachs with contents were placed in plastic bags, and stored frozen within about 2 h of capture. The stomach contents were weighed, fixed in 4% buffered formalin for 24 h, and preserved in 70% alcohol. The items were carefully separated, weighed (after removing the surface water by blotting with tissue paper) and identified to the lowest taxonomic level possible. The individuals of each identified taxon were counted. Whenever fragments were found, the number of individuals was determined as the lowest possible number of individuals from which fragments could have originated. DIET ANALYSIS.—The method of cumulative trophic diversity (Hurtubia, 1973), measured by the

Shannon index (as H'= -S Pi Log Pi, where Pi is the proportion of individuals in the ith species), was used to determine the adequacy of the sample size (Mauchline and Gordon, 1985). 1076 BULLETIN OF MARINE SCIENCE, VOL. 69, NO. 3, 2001

Three indices were used to describe the diet, the frequency of occurrence (%O), percentage numbers (%N) and weight (%W) (methods reviewed by Hyslop, 1980). Wet weight was used to determine the latter value, after removal of superficial water from each item with soft tissue paper (Glen and Ward, 1968). The Index of Relative Importance

IRI=+()%% N W¥ % FO as proposed by Pinkas et al. (1971) was calculated for each prey category. For diet comparisons, the %IRI for each food category

n %IRIiii=¥100 IRIÂ IRI i=1 was used after grouping prey taxa into categories based on their taxonomy and ecology. Ontogenetic differences in diet of the blackspot seabream were examined by grouping fish lengths in size classes (14–24.9, 25–34.9 and 35–47.0 cm FL), which were arbitrarily assumed to express shifts in prey composition. Diets were also analyzed by sex, areas (coastal and offshore banks), as well as depth range (0–200, 201–400, 401–600 m). The axillary seabream samples could not be stratified and analyzed due to the small number of stomachs with contents. Schoener’s dietary overlap index (Schoener, 1970) (as Cxy= 1-0.5S|Pxi - Pyi|, where Pxi was the proportion—based on

%IRI—of food category i in the diet of x; Pyi was the proportion of food category i in the diet of y) was used to measure the diet overlap between size classes, depth strata, sex, and between species. This index was also used to measure the overlap between the overall diet of axillary seabream and the different blackspot seabream size classes. STATISTICAL ANALYSIS.—Differences in the rankings of IRI values for prey categories (i.e. Gas- tropoda, Bivalvia, Cephalopoda, Crustacea, Ophiurida, Thaliacea, and Pisces) between three or more groups (e.g., three size classes) were tested for significance with Kendall Coefficient of Con- cordance (w) (Siegel and Castellan, 1988). For paired groups (e.g., males and females), the Spearman rank correlation (rs) was used (Fritz, 1974; Zar 1996).

RESULTS

Blackspot seabreams were caught over a bathymetric range of 0 to 700 m, but mostly between 100 and 400 m depth (Fig. 2). Of 1263 individuals examined, 559 had only bait in their stomachs (44.2%), while 592 (46.9%) had empty stomachs. A total of 112 individuals (8.9%) had stomachs with contents. The fishes with food present ranged from 14.0 to 47.0 cm FL. More males (76.5%) than females (11.7%) were caught during the longline experiments. Axillary seabreams were captured between 0 and 400 m depth, but the majority were from the first 100 m (Fig. 2). From the 235 axillary seabreams examined, bait only was found in the stomachs of 79 individuals (33.6%), while 118 (50.2%) had empty stomachs. A total of 38 individuals (16.2%) had stomachs with contents. The sizes of the fish with food varied from 21 to 28 cm (FL). More females (72.3%) than males (26.0%) of axillary seabream were sampled. OVERALL DIET COMPOSITION.—Pagellus bogaraveo.—Recognizable prey from over 19 different taxa were identified from the 305 prey found in the stomachs of blackspot seabream. From the axillary seabream stomachs, 158 prey were identified belonging to MORATO ET AL.: FEEDING HABITS OF TWO CONGENER SPECIES OF SEABREAMS 1077

Figure 2. Frequency distributions of fishes caught, by depth range.

13 different taxa. Cumulative trophic diversity curves (Fig. 3) assumed asymptotic values quickly, with about 30 stomachs analyzed for each species. The diet of blackspot seabream consisted of a wide variety of organisms dominated by fishes, which occurred in 61.3% of all stomachs with food and represented 66.0% of its weight (data not presented). The principal fish prey found were myctophids, including unidentified species (IRI = 214.0), Ceratoscopelus maderensis (IRI = 5.6) and Lobianchia dofleini (IRI = 0.6). Other fish species included Macroramphosus scolopax (IRI = 113.2), Sternoptychidae (IRI = 6.0), and Capros aper (IRI= 4.7) (Table 1). Blackspot seabream also fed on several invertebrates such as thaliaceans (IRI = 1945.0) and ophiuroids (IRI = 198.8), which represented 29.2% and 17.7% of all food items counted and occurred in 37.8% and 9.9% of all stomachs, respectively. Only one species of gastropods was identified, Diacria trispinosa (IRI = 35.4), occurring in 6.3% of examined stomachs. Two cephalopods (IRI = 7.9) were also found in stomachs of the blackspot seabream. 1078 BULLETIN OF MARINE SCIENCE, VOL. 69, NO. 3, 2001

Figure 3. Variation of cumulative trophic diversity of blackspot and axillary seabreams diet with sample size.

Pagellus acarne.—The diet of axillary seabream was also dominated by fish species, accounting for 84.3% of total contents by weight, and occurring in 76.3% of all stomachs analyzed (data not presented). The principal fish prey found were—M. scolopax (IRI = 456.8), and C. aper (IRI = 222.7) as well as myctophids (IRI = 27.5) (Table 2). Several invertebrates also constituted part of the diet. Thaliacea (IRI = 262.2), occurred in 21.1% of all stomachs, and ophiuroids (IRI = 220.1), accounted for 38.6% of all food items counted, and were the most important invertebrate taxa found. Echinoids, mainly Abraciella elegans (IRI = 46.1), bivalves and some decapods were also identified in the stomach contents of this species. DIET COMPOSITION BY FISH SIZE.—Diet composition was relatively homogeneous among blackspot seabream size groups (14–24.9 cm, 25–34.9 cm and 35–47.0 cm FL) with fishes and thaliaceans being the main prey categories. Based on IRI values no significant differences (w = 0.76, P < 0.0245) were found for prey composition by fish length. Schoener’s overlap index also indicated a high degree of diet overlap (>0.70) between all size classes. However, an apparent increase in consumption of myctophids and a declin- ing of ophiuroids was observed with increasing fish length (Table 3). The consumption patterns of thaliaceans and ophiuroids with fish size are not clear. DIET COMPOSITION BY SEX.—For blackspot seabream, the Spearman rank correlation of

IRI values calculated for males and females was not significant (rs = 0.58, P < 0.127) indicating that the overall diet of the sexes was not similar (Fig. 4). Accordingly, Schoener’s index indicates a low overlap (0.28) between the diets of females and males. Prey items such as myctophids and D. trispinosa were more important to the diet of females, while thaliaceans, ophiuroids, and the fish species M. scolopax were mostly consumed by males (Table 3). DIET COMPOSITION BY DEPTH RANGE.—Multiple correlation of IRI values were poorly significant (w = 0.68, P < 0.0451) when comparing the diets of the blackspot seabream MORATO ET AL.: FEEDING HABITS OF TWO CONGENER SPECIES OF SEABREAMS 1079

Table 1. Number (n), percent number (%N), percent weight (%W), number of predators with one item (nP) frequency of occurrence (%O) and Index of Relative Importance (IRI) calculated for each prey item found in the stomachs of blackspot seabream, Pagellus bogaraveo.

InNtems %W%PnO%IIR A15lgae 00. 019. 04. 0. P15orifera 07. 019. 00. 1. H15ydrozoa 00. 019. 04. 0. S20iphonophora 11. 028. 18. 1. Gastropoda Diacria trispinosa 131 54. 073. 64. 35. B20ivalvia 10. 028. 18. 1. C20ephalopoda 15. 328. 19. 7. A15rthropoda unidentified 00. 019. 04. 0. Crustacea P15enaeidea 05. 019. 09. 0. A15mphipoda 01. 019. 05. 0. C49rustacea unidentified 17. 246. 35. 16. B15ryozoa 00. 019. 05. 0. O7phiurida 37147. 21. 1998. 198. T1haliacea 62229. 222. 48307. 1,945. Pisces S20ternoptychidae unidentified 14. 228. 10. 6. T0otal Myctophidae 2698. 137. 17121. 320. Ceratoscopelus maderensis 2011. 228. 16. 5. Ceratoscopelus s34p. 14. 228. 19. 6. Lobianchia dofleini 1502. 019. 06. 0. M4yctophidae unidentified 1761. 123. 18100. 214. Gadella maraldi 1504. 019. 08. 0. Capros aper 1508. 419. 07. 4. Macroramphosus scolopax 7436. 1734. 62. 113. P0isces unidentified 59293. 265. 44481. 2,062. T5otal 300 100 110 181 137.

caught at different depths. This means that no significant differences were found in their diet composition. However, Schoener’s overlap index reveals a high level of overlap between the two shallower depth strata (0–200 vs 201–400; 0.77) and a low overlap between the second and the third depth strata (201–400 vs 401–600; 0.23). Diet shift may occur at 400 me depth (Table 4). Consumption of thaliaceans and the fish species M. scolopax decreased with increasing depth (Table 4), while an increasing in the consumption of myctophids and D. trispinosa occurred with depth. Ophiuroids were mainly consumed at depths below 200 m. DIET COMPOSITION BY AREA OF CAPTURE.—For blackspot seabream, rankings of IRI values for coastal areas and offshore banks were significantly correlated (rs = 0.73, P = 0.0390). Accordingly, a high level of diet overlap was found (0.87) between the individuals caught at the offshore banks and at coastal areas. However, some differences were observed for the less important prey categories. Ophiuroids and cephalopods were only found for blackspot seabream caught in coastal areas. Several other items, such as myctophids, thaliaceans and the gastropod, D. trispinosa, have similar importance in 1080 BULLETIN OF MARINE SCIENCE, VOL. 69, NO. 3, 2001

Table 2. Number (n), percent number (%N), percent weight (%W), number of predators with one item (nP) frequency of occurrence (%O) and Index of Relative Importance calculated for each prey item found in the stomachs of axillary seabream, Pagellus acarne.

InNtems %W%PnO%IF IR S23ipunculida 11. 11. 22. 66. B97ivalvia 57. 09. 203. 7152. Parvicardium s16p. 01. 01. 29. 61. B81ivalvia unidentified 56. 08. 201. 1120. P45olychaeta 24. 13. 76. 930. Crustacea C16aridea 01. 01. 29. 61. N81atantia unidentified 53. 01. 22. 614. B39rachyura 19. 03. 73. 922. I23sopoda 15. 02. 55. 39. C39rustacea unidentified 10. 01. 21. 65. B16ryozoa 02. 01. 21. 62. E0chinoidea unidentified 1366. 04. 180. 572. Arbaciella elegans 4529. 14. 110. 546. O1phiurida 66328. 32. 51. 3220. T2haliacea 1679. 48. 221. 1262. Pisces M39yctophidae 16. 81. 25. 627. Capros aper 4527. 2395. 77. 222. Macroramphosus scolopax 8157. 2529. 183. 456. P3isces unidentified 26144. 230. 25660. 2,115. T8otal 105 1000. 1800. 35210.

both areas (Table 4). Several fishes, such as Sternoptychidae and Gadella maraldi, were found only at the offshore banks. DIET OVERLAP BETWEEN PAGELLUS BOGARAVEO AND PAGELLUS ACARNE.—The overall diet overlap between the two studied species was relatively high (0.59) indicating, at some extent, the exploitation of similar prey (Table 5). However, the high importance, in terms of %IRI, of unidentified fishes may contribute to an artificial increase of overlap (see Table 2,3). Diet overlap between this two species decrease with the increasing of blackspot seabream size. Thus, being higher for the small size class (0.68), than for the larger size group (0.45).

DISCUSSION

Several authors have discussed the influence of the fishing gear in studies of feeding habits of fish. Morato et al. (1999) and Wirtz and Morato (2001) have focused the Azorean case and concluded that in spite of all the problems caused by the use of longline, the demersal cruise surveys are the only way to obtain abundant biological data for growth, reproduction, food habits and genetic studies, of several fish species in the Azores. The Shannon index used in the accumulative trophic diversity achieved asymptotic values. This indicated that the diets of both species are adequately defined in terms of trophic diversity and ecological constitution with relatively low intensity sampling. MORATO ET AL.: FEEDING HABITS OF TWO CONGENER SPECIES OF SEABREAMS 1081

Table 3. Percentage of Relative importance (%IRI) of food categories consumed by Pagellus bogaraveo of each of the three length groups, and by females (F) and males (M). Schoener's diet overlap index is also presented.

Txotal length (cm) Se 14-254.9 2 -354.9 3 -4FM7.0 A6lgae 00.0 00.0 00.0 00.0 0.0 P0orifera 08.0 00.0 06.0 10.6 0.0 H6ydrozoa 00.0 00.0 00.0 00.0 0.0 S0iphonophora 06.0 00.1 00.0 07.0 0.0 Diacria trispinosa 05.00 30.1 05.0 68.4 0.2 B6ivalvia 00.2 00.0 00.0 02.0 0.0 C1ephalopoda 10.9 00.0 00.0 03.0 0.3 A0rthropoda 04.0 00.0 00.0 02.0 0.0 P9enaeidea 00.1 00.0 00.0 04.0 0.0 A0mphipoda 04.0 00.0 00.0 02.0 0.0 C0rustacea unidentified 04.0 10.3 04.0 51.0 0.2 B0ryozoa 04.0 00.0 00.0 02.0 0.0 O6phiurida 63.5 20.5 00.0 00.0 4.4 T8haliacea 345.0 345.2 446.5 172.7 43.2 S0ternoptychidae 08.0 00.4 00.0 04.0 0.2 Myctophidae1 29.20 75.5 116.1 611.4 1.7 Gadella maraldi 06.00 00.0 00.0 03.0 0.0 Capros aper 06.00 00.3 00.0 00.0 0.0 Macroramphosus scolopax 09.21 70.0 00.0 02.0 1.5 P5isces unidentified 593.4 411.7 307.3 132.7 47.8

Schoener's diet overlap

S9tomachs with contents 385412197 1 the combined percentage of all Myctophidae species Both seabreams studied fed on a wide variety of small organisms, mainly fishes and several invertebrates. The most important fish species were the mesopelagic myctophids for the blackspot seabream and the benthopelagic M. scolopax and C. aper for the axillary seabream. Thaliaceans and ophiuroids were the most important invertebrates for both species, however blackspot seabream also fed on pelagic gastropods while axillary seabream consumed bivalves and echinoids. The majority of the prey consumed by both predators live in benthopelagic and benthic environments. Therefore, it is apparent that they are not dependent on live bottom dwelling organisms for prey, since they can feed on benthopelagic, pelagic and even mesopelagic prey. However, it is clear that the axillary seabream is more dependent on bottom dwelling organisms than the blackspot seabream, being a result of differences in habitat preferences. These results when interpreted in the seamount context, which is the major topographic feature of the Azores region, can help to elucidate how seamounts support large fish communities. As showed before, both species feed on benthic prey species, which are 1082 BULLETIN OF MARINE SCIENCE, VOL. 69, NO. 3, 2001

Figure 4. Relative importance of prey categories in the diet of Pagellus bogaraveo, ranked from highest IRI values for males and males; where, %N is the percent number, %W the percent weight and %O the frequency of occurrence of the prey category. Each tick mark of %O represents 10%. locally produced. This source of energy represents 65.8% and 10.2% of total food in- gested by axillary and blackspot seabream, respectively, during the spring (Table 6). However, the other important component of their diets are pelagic species, which may be trapped around seamounts and intercepted during their diurnal migration (see Rogers, 1994; Koslow, 1997). This could be the case of some myctophids, which represents 14.4% of food consumed by the blackspot seabream. Other sources of food could be provided by drifting organisms that pass seamounts with the ambient current, which could be the case of pelagic gastropods, salps, and other drifting organisms. This source of energy represents 67.1 and 5.5% of the total food consumed by the blackspot and the axillary seabream, respectively. It is clear that in addition to locally produced food, the productivity of many MORATO ET AL.: FEEDING HABITS OF TWO CONGENER SPECIES OF SEABREAMS 1083

Table 4. Percentage of relative importance (%IRI) of food categories consumed by Pagellus bogaraveo in four depth strata, and in two areas: coastal and off shore banks. Schoener's diet overlap index is also presented.

Dsepth (m) Area 0-2100 20 -4100 40 -6l00 Csoasta Bank A3lgae 00.0 00.0 02.0 00.0 0.0 P0orifera 04.0 00.2 00.0 04.0 0.1 H3ydrozoa 00.0 00.0 02.0 00.0 0.0 S3iphonophora 01.0 00.1 00.0 00.0 0.3 Diacria trispinosa 01.03 08.1 173.2 08.7 0.6 B3ivalvia 00.0 00.1 09.0 00.0 0.0 C8ephalopoda 00.4 00.0 02.0 00.5 0.0 A0rthropoda 00.0 07.0 02.2 00.0 0.0 P0enaeidea 01.0 00.2 06.0 00.0 0.0 A0mphipoda 01.0 00.1 00.0 08.0 0.0 C4rustacea unidentified 01.0 15.3 08.4 04.3 0.2 B3ryozoa 00.0 00.0 00.0 07.0 0.0 O4phiurida 06.2 171.2 43.4 70.7 0.0 T7haliacea 499.8 248.8 33.9 356.6 42.9 S3ternoptychidae 08.0 00.5 00.0 06.0 0.7 Myctophidae1 19.03 41.6 678.6 85.3 5.2 Gadella maraldi 00.05 00.0 00.0 01.0 0.1 Capros aper 08.00 10.0 02.0 00.3 0.0 Macroramphosus scolopax 32.34 20.3 08.0 08.7 5.6 P4isces unidentified 484.7 488.9 80.9 454.3 43.7

Schoener's diet overlap

S6tomachs with contents 583711714 1 the combined percentage of all Myctophidae species

seamounts communities is also dependent on a regular supply of productivity that drift past seamounts. Both species are known for displaying vertical feeding migrations as described by Desbrosses (1983) and Uranga (1990) for blackspot seabream, and by Domanevskaya and Patokina (1984) for axillary seabream. The occurrence of migrations can be inferred by the presence of some mesopelagic and pelagic prey, and would support the theory of fish ascending 100s of meters into the water column over the seamount to feed on trap- ping or drifting prey. However, our results could not clarify daily vertical migrations of these predators. Thus, sampling over a 24-h period could be necessary to fully understand the daily feeding activity of these predators. Our results concerning the diet composition of the blackspot seabream are similar to those presented by Olaso and Pereda (1986) for the Cantabrian Sea. These authors did not present any detailed list of prey occurrence, hence disabling us to make any further com- ment on habitat and geographic differences in the diet of this species. The diet of the Azorean axillary seabream was slightly different from the one presented by Olaso and 1084 BULLETIN OF MARINE SCIENCE, VOL. 69, NO. 3, 2001

Table 5. Schoener's diet overlap index between axillary seabream (Pagellus acarne) and blackspot seabream (Pagellus bogaraveo).

Pagellus bogaraveo A)ll Shize classes (cm FL Dept 14-204.9 25. -354.9 3 -407.0 -200 Pagellus acarne (21.0-298.0 cm FL) 08.5 01.6 05.6 07.4 0.5

Rodrígues-Marín (1995) in the Cantabrian sea and Andoloro (1982, 1983) in the Tyrrhenian and Ionian Sea. These authors reported that fish were relatively rare, while crustaceans, mainly decapods, as well as polychaetes dominated the diet. Differences between these two studies may be attributed to differences in the environment from where samples originated and regional differences in fauna composition. The high consumption of fish in the Azores compared to that in the Cantabrian Sea, may be a result of the high productivity of seamounts as well as the localized fish prey aggregation around them. Other sources of differences could be related to the depth of capture, size of fish and time of the year. Olaso and Rodrígues-Marín (1995) used a greater size range of fish and sampled only from September to October, which may also contribute to the observed differences. The main components of the blackspot seabream diet did not vary significantly with fish length. However, the evidence of increasing consumption of myctophids, and the decreasing consumption of ophiuroids according to length of blackspot seabream, might be related to several parallel factors, such as sex and depth of capture. Examining the sex- related and depth related differences, one can observe that males consumed more ophiuroids, thaliaceans and the fish species M. scolopax at shallower waters, where females consumed more myctophids and the pelagic gastropod D. trispinosa, which are heavily preyed upon in deeper waters. As it was stated early in the paper, the blackspot seabream is a protandric hermaphrodite with sex inversion occurring between 27 and 34 cm FL (Krug, 1990). Thus, the diet of males and females is size and depth dependent. Krug (1994) showed that males are small individuals that occur more frequently in coastal areas, while females are usually bigger and are more frequently found at the Azorean offshore banks. This suggests a migration of juveniles from coastal areas to offshore banks. This theory is confirmed by our stomach content analysis. Our stomach contents analysis also suggests a depth related segregation of the blackspot seabream. Small seabreams, which live preferentially in coastal and shallower waters, and are essentially males, have different feeding habits from those observed for larger individuals, which live preferentially at offshore banks at

Table 6. Percentage of relative importance (%IRI) of ecological categories of prey consumed by blackspot seabream (Pagellus bogaraveo) and axillary seabream (Pagellus acarne).

Amxillary seabream Blackspot seabrea Z2ooplankton 00. 0. B8enthic invertebrates 625. 10. B5enthic decapods 00. 0. P0elagic invertebrates 00. 2. P5elagic salps 51. 67. B5enthopelagic fish 207. 6. M6esopelagic fish 04. 14. MORATO ET AL.: FEEDING HABITS OF TWO CONGENER SPECIES OF SEABREAMS 1085

deeper waters and are mainly females. We concluded that the differences in blackspot seabream diet are due to the use of different environments and consequently, prey availability. There also appears to be a preference for larger prey as the predator grows. How- ever, further studies are necessary to clarify this aspect. The overall differences reflect the opportunistic foraging behavior of both species and suggest that it is difficult to make generalizations about feeding habits of these two species. It seems likely that the observed patterns are determined by the responses of fish to habitat-specific features, such as assemblages of prey and predators, as well as by the absolute and relative abundance of potential food items in the environment. However, information regarding the abundance of prey organisms in the Azores environment is not available, and therefore it is not clear whether the two study species select their prey preferentially or exploit the food resources based upon their overall abundance in the specific habitat where they occur. Consideration of both distribution and food habits indicates differences in resources use between the two Pagellus species. Although the results of the stomach contents analysis showed a significant feeding overlap, we suggest that they may not be sharing resource the trophic activities of the two species are segregated by differences in habitat use. Blackspot seabream is more common between 100 and 400 m depth while the axillary seabream were mainly found down to100 m (Fig. 2). Moreover, the axillary seabream has never been found at offshore banks, where the blackspot seabream is quite common (Menezes et al., 2001). However, there may be an interaction between small individuals of both species in shallow waters, since they exhibited a significance habitat and diet overlap. The niche complementary hypothesis (Ebeling and Hixon, 1991) implies that co-occurring species that show a high degree of overlap in one niche dimension (diet) separate out along another dimension (habitat). In this study, habitat partitioning appeared to be an important factor in preventing diet overlap.

ACKNOWLEDGMENTS

This work is part of a more comprehensive study supported by the European Union (Design optimisation and implementation of demersal cruise survey in the Macaronesian Archipelagos. Study contract DG XIV/94/034 and DG XIV/95/095). Thanks are due to J. Gonçalves, R. Serrão Santos, F. Porteiro for help with identification of stomach contents, and Helena Krug for help with identification of otoliths. Special thanks are due to the scientific staff and to the crew of the RV ARQUIPÉLAGO for working overtime at sea. We are also grateful to P. Afonso, H. Martins and N. Carvalho for reviewing the manuscript.

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DATE SUBMITTED: December 30, 1999. DATE ACCEPTED: March 7, 2001.

ADDRESSES: (T.M., E.S., M.P.G., G.M.) Departamento de Oceanografia e Pescas, Universidade dos Açores, PT-9901-862 Horta, Portugal. CORRESPONDING AUTHOR: (T.M.) E-mail: .