original scientific article	UDC 639.23:597.5(262.3-11 )
received: 2005-09-06
SEASONALITY IN DIEL CATCH RATE OF LABRIDS IN A SHALLOW-WATER HABITAT AT DUCE GLAVA BEACH IN THE EASTERN ADRIATIC
Sanja MATIČ-SKOKO, Jakov DULČIČ, Miro KRALJEVIČ & Livija TOMASOVIČ Institute of Oceanography and Fisheries, HR-21000 Split, P.O. BOX 500, Croatia E-mail: sanja@izor.hr
ABSTRACT
An assemblage of shallow-water fishes was sampled with a small beach seine monthly, during 24-hour periods, between April 2000 and March 2001 at the sandy Duce Glava beach in the eastern Adriatic. Monthly sets of samples were divided into day and night catches to examine the stability of diel differences in assemblage structure over a one-year period. A total of 284 Labrids, mainly juveniles, representing 6 species (Symphodus ocellatus, S. cinereus, S. roissali, S. rostratus and Labrus viridis, L. merula) were sampled during the whole investigation period. Each of these Labrid species had a different pattern of diurnal and seasonal abundance. S. ocellatus was the dominant species throughout the year. The highest increases in number of all Symphodus species were recorded in the summer period. L. viridis, represented by the largest individuals of all sampled Labrids, were not found during the summer and autumn months. S. ocellatus, S. roissali and S. rostratus appeared to be mainly diurnal, while at night more S. cinereus and L. viridis individuals were caught.
Key words: seasonality, day/night differences, Labrids, Adriatic Sea, coastal area
STAGIONALITA Dl CATTURE GIORNALIERE Dl LABRIDI Dl ACQUE POCO PROFONDE IN MARE ADRIATICO
SINTESI
Una comunita ittica di acque poco profonde e stata campionata mensilmente per 24 ore, con l'ausilio di una piccola rete da pesca, tra aprile 2000 e marzo 2001, lungo la costa sabbiosa Duce Glava nell'Adriatico orientale. I campioni mensili sono stati divisi in catture diurne e notturne, al fine di esaminare la stabilita delle differenze gior-naliere nella struttura della comunita, durante il periodo di un anno. Un totale di 284 labridi, in prevalenza stadi giovanili di 6 specie (Symphodus ocellatus, S. cinereus, S. roissali, S. rostratus, Labrus viridis e L. merula), sono stati campionati durante l'intero periodo di studio. Ognuna di queste specie di labridi ha mostrato di avere un andamento diverso dell'abbondanza giornaliera e stagionale. S. ocellatus e stata la specie dominante durante tutto l'anno. L'incremento maggiore nel numero di specie di Symphodus si e registrato durante il periodo estivo. L. viridis, rappre-sentato dagli esemplari piu grandi fra tutti i labridi campionati, non e stato trovato durante restate e l'autunno. S. ocellatus, S. roissali e S. rostratus sono apparse specie prevalentemente diurne, mentre la gran parte degli individui di S. cinereus e L. viridis e stata catturata durante la notte.
Parole chiave: stagionalita, differenze giorno/notte, labridi, mare Adriatico, area costiera
INTRODUCTION
Several studies have emphasized the role of marine shallow-water habitats (Thiel et al., 1995; Biagi et al., 1998; Nash & Santos, 1998; Guidetti & Bussotti, 2000) as nurseries of a variety of marine fishes. It has been shown that fish assemblage changes over diel periods (Nash, 1986; Wright, 1989; Nash et al., 1994) due to the net avoidance during the daytime (McCleave & Fried, 1975) also due to the real changes in abundance and the assemblage structure (Lasiak, 1984; Nash et al., 1994). Moreover, there is a seasonal change in the species composition, which is also reflected in the diel variations in assemblage structure (Nash, 1986; Wright, 1989).
Only a limited number of previous studies have examined the day/night catches for a whole year (Allen et al., 1983; Nash & Santos, 1998; Dulcic et al., 2004). Diel changes in assemblage composition, which are then superimposed on seasonal changes, could have a profound effect on the perception of a fish assemblage. The diel periodicity of an assemblage, or even the lack of periodicity, is caused by changes in catch of the individual species (Nash & Santos, 1998).
Wrasses (the family Labridae) are the most abundant
and conspicuous fishes on tropical reefs around the world (Choat & Bellwood, 1998). They also comprise an important element of the coldwater fish population in temperate areas, such as Adriatic Sea, and can be found in a wide variety of habitats (Jardas, 1996). Wrasses appear in a diverse range of colours, shapes, and sizes, often varying considerably within species. They belong to the productive and low trophic level species with high ecological efficiency (Dulcic et a/., 1997; Wainwright & Bellwood, 2002). Many wrasses are organized into harem-based social systems and hermaphroditism is common (Choat & Bellwood, 1998).
Eighteen species from the Labridae family are found in Adriatic Sea (Jardas, 1996). The Symphodus spp. is one of the most abundant genera of Adriatic Labrids, while other species are rarely encountered. Although these species are not commercially interesting, they occupied considerable place in the Adriatic ichthyofauna due their pronounced biodiversity. Labrids are of tropic origin, but due to climate change, future appearance of new Labrid species, together with the increasing abundance of present Labrids as well as their movement to the northern parts of the Adriatic Sea, is expected (Dulcic & Grbec, 2000).
Fig. 1: Map showing the area of Duce-Glava Bay and the Cetina estuary. SI. 1: Zemljevid z zalivom Duce Glava v ustju reke Cetine.
RESULTS
Mean daily temperature over the year (2000-2001) ranged from 12.3 °C in February 2001 to 23.0 °C in July 2000, while salinity ranged from 27.7 in February 2001 to 35.3 in July 2000.
This study analyses the day/night changes in catches of shallow-water Labrids over one year period and provides: 1) a description of temporal and seasonal fluctuations of Labrids in a shallow-water with mixed habitats, and 2) a comparison with other subtropical and temperate studies. Results obtained in this study could be useful for gaining a better understanding of Labrid fish community structure in the coastal Adriatic area.
MATERIAL AND METHODS
Duce Glava is a small south-facing sand beach on the Croatian Adriatic coast (approximately 20 km south of Split) near the Cetina river estuary (Fig. 1). The sampling area was sandy and partially overgrown with meadows of Posidonia oceanica. The beach is a popular swimming area during the summer.
Samples were collected monthly for 11 consecutive months (April 2000-March 2001). Due to exceptionally bad weather, sampling was not possible in January 2001. Each month, 7 samples were taken at 4-hour intervals with a 22 m beach seine (wings of 7.5 m and central collecting area of 7 m, 4 mm stretch size mesh at the wings reducing to 2 mm in the centre) at depths from 0.1 to 1.5 m. Before each sample, water temperature was measured and is presented as the average of the 7 measurements taken in each 24-hour sampling period. Each sample represents two sets of the net. Temperature was measured with a mercury thermometer, salinity with a laboratory inductive salinometer before each sampling.
Fish were immediately preserved in 4% formaldehyde and identified using Jardas (1996). The total number of individuals and total weight for each species in each haul was obtained. Total lengths (to the nearest 0.1 mm) and individual weights (to the nearest 0.01 g) were measured for each sample. Juvenile fishes were defined as specimens with already formed scales, and were taken as such until the moment of first sexual maturity (Katavic, 1984).
The fish data were analysed using the PRIMER software package (Plymouth Marine Laboratories, UK; Clarke & Warwick, 2001). Data were transformed for presence/absence and the Bray-Curtis similarity matrix was used to generate 2-dimensional ordination plots with the non-metric multidimensional scaling (nMDS) technique (Clarke, 1993). ANOSIM test for the two-way crossed analysis was used for testing differences in species assemblage between seasons and hours (Clarke & Warwick, 2001). Probability value was set at 0.05. Species presented in total sample with less than 5% were omitted from this analysis.
Fig. 2: Labrid representation among length classes at Duče, Adriatic coastal area.
Sl. 2: Ustnače zaliva Duče, razvrščene po različnih velikostnih razredih.
Labrids comprised 284 individuals (1.6%) of total catch (17,414 fish representing 61 species). The Labrid species were: Symphodus ocellatus (68.2% of total Labrids), Symphodus cinereus (20.1%), Symphodus rois-sali (8.5%), Symphodus rostratus (1.8%) and Labrus viridis (1.4%). Also, one individual of Labrus merula (Lt = 6.7 cm; Wt = 4.04 g in June at midnight) was found but is, as a single specimen, omitted from further analysis. A Labrid representation among length classes is presented in figure 2. Mostly, S. ocellatus individuals belong to middle-length classes from 4.0 to 7.0 cm. S. rois-sali were uniformly distributed through different length classes. The higher number of longer S. cinereus individuals from 7.0 to 11.0 cm were recorded, while the longest individuals were those of L. viridis belonging in length classes between 12.0 and 15.0 cm with the mean length value of 13.9 cm.
Each of these Labrid species had a different pattern of diurnal and seasonal abundance. Number of Labrids caught per season was the highest in summer (172); it declined in autumn (57), and was the lowest in winter (14). In spring, 40 Labrids (14.1%) were sampled. S. ocellatus were the most dominant Labrid species (42.5%) in spring, while S. roissali individuals were not observed in that period. The highest increase in the number of all Symphodus species was recorded in the
summer period. This trend was specially pronounced for S. ocellatus with 193 caught specimens. L. viridis was not found at all during summer and autumn. Lower sea temperatures were accompanied by number declination of Labrid species in the Duce area. Seasonal variability was the highest in spring and summer, although there was a large fluctuation in the number of species between those months (Fig. 3). S. ocellatus individuals were dominant between April and October. Higher abundance of S. roissali specimens was recorded from April to August. S. cinereus individuals were almost uniformly distributed throughout the year. The highest number of S. rostratus individuals was sampled in June.
The nMDS plots of Labrid samples (Fig. 4) shows there is some difference between them in colder and warmer periods, especially during the spring and winter months. Catches of the analyzed Labrid species showed little variation during the winter. Results of the ANOSIM test for the two-way crossed analysis showed that there was a significant difference in species assemblages among seasons, although correlation was not high (pav = 0.118, P = 0.017).
Fig. 3: Monthly distribution of Labrids at Duce, Adriatic coastal area.
Sl. 3: Mesecna razporeditev ustnac v zalivu Duce.
Over the entire year, slightly more Labrids were caught at day (152) than at night (130). With respect to the number of individuals, S. ocellatus, S. roissali and S. rostratus catches appeared to be mainly diurnal. On the other hand, more S. cinereus and L. viridis individuals were found in night samples. Diurnal differences in occurrence were more pronounced in S. roissali (16/day and 8/night) and S. cinereus (19/day and 38/night) species. S. ocellatus, as the most dominant species, was also recorded with the highest catch in each of the sub-samples (Fig. 5.). S. roissali was not found in samples
taken at midnight while, at the same time, the highest number of S. cinereus was recorded.
The nMDS plots of Labrid samples (Fig. 6) shows there is a clearly visible overlapping of the day and night samples. Moreover, the result of ANOSIM test for the two-way crossed analysis showed that there was no significant difference in those catches between day and night samples (pav = 0.118, P = 0.017).
DISCUSSION
Differences in day/night composition of fish catches can have a profound effect on the perception of a fish community (Nash & Santos, 1998). Temporal variation in species abundance pointed out species separation and a partitioning of the habitat along a time axis. The advantage of this time distinguishing is reduction in competition for food and/or space and avoidance of predation (Ross, 1986).
The qualitative dominance of Sparids and Labrids and the quantitative dominance of schooling fish species in the shallow rocky Mediterranean habitats are well known (Guidetti, 2000). The preference by Labrids for substrates colonised by macroalgae has not been known only for the Mediterranean (Garcia-Rubies & Macpher-son, 1995), but is a common feature in all temperate waters (Choat & Ayling, 1987). In both papers, it was affirmed that dense algal stands in shallow waters are often colonised by young Labrids, which use the algae for feeding and shelter. Moreover, Guidetti (2000) found that in the Adriatic S. ocellatus is one of the most common species in the P. oceanica seagrass beds, while S. roissali and S. tinca were mainly associated with the rocky-algal reef habitats. Structure of fish assemblages associated with some habitats, and thus occurrence of some species, is affected by a large number of inter-playing and, in some cases, superimposed biological interactions (i.e. grazing, predation) and physical factors (i.e. habitat complexity, hydrodynamic forces). The differences in fish species richness and abundance are primarily related to habitat structure (Guidetti, 2000). According to fact that the area of Duce was sandy and partially overgrown with meadows of P. oceanica, it was expected to have more Symphodus species, which prefer this type of habitats, than other Labrids. The substrate type (Levin, 1991) and depth (Garcia-Rubies & Mac-pherson, 1995) are two main factors affecting fish recruitment, mortality and growth. Although, Letourner et al. (2003) found that bottom slope was generally more significant than depth. Furthermore, Letourner et al. (2003) suggested that benthic habitat use by fish is clearly non-random, and that differences in habitat at a small spatial scale can affect fish assemblages.
Fig. 4: Non-Metric Multidimensional Scaling (nMDS) ordination plot for Labrids during each season (stress < 0.01): Xspring, Z summer, aautumn, awinter. Sl. 4: Nemetrično multidimenzionalno skaliranje (nMDS) za ustnače v posameznih letnih časih (pomen < 0,01): X pomlad, Z poletje, ajesen, azima.
Symphodus species found in the sampling area were small Labrids (cca. 10 cm), while those from Labrus spp. can reach lengths of up to 30 cm (Jardas, 1996). All caught specimens were in the length classes from 2.0 cm to 15.0 cm. The most frequent lengths of occurrence of the analysed Labrids are consistent with the data reported by Jardas (1996). Labrids belonged to resident species that, after metamorphosis, live permanently in the same area, where they also breed (Guidetti & Busotti, 2000; Dulčic et al., 2002). Some temperate wrasse species, such as the S. ocellatus, S. cinereus and S. rostratus, are demersal nest builders. The nests are usually made of plant material and the male guards the eggs after they are deposited (Jardas, 1996). The time periods of occurrence of the analysed Labrid juveniles are consistent with the data reported by Dulčic et al. (1997). The highest abundance of all specimens was recorded during the summer (June) and the lowest during the winter (December). S. ocellatus were dominant Labrid species in the investigated area. Its juveniles were also one of the four dominant species in the Kornati Archipelago (Dulčic et al., 1997). All species were more numerous during the summer, except L. viridis, which was equally found during the spring and winter periods. Discussion about S. rostratus, L. viridis and L. merula differences in day/night catches do not have so much sense owing to their minor percentage in total sample. One of the major reasons for such Symphodus spp. occurrence was in the recruitment timing of these species. Moreover, a high abundance in June and July was due to the increase of their juveniles, and this could be correlated
with the spring spawning period (Jardas, 1996) and duration of embryonic development of these species (Dulcic et al., 1997). It seems that in tropical wrasses spawning occurs year-round, while some temperate species obviously restrict spawning to warmer parts of the year (Choat & Ayling, 1987).
Specimens of Symphodus spp. were found almost during all months, but with higher abundance from June to October. Biagi et al. (1998) observed settlers of S. cinereus for only one or two months in the 0-3 m zone of the Italian costal area, and they supposed that after that they probably moved offshore to deeper waters. On the other hand, settlers of S. rostratus were observed at the samples for an extended period after settlement occurred (Biagi et al., 1998), which is in agreement with our results. Segregation of the analysed species, in terms of reducing the possible interspecific competition for food and shelter, was not observed.
Fig. 5: Labrids representation between different sampling hours at Duče, Adriatic coastal area. Sl. 5: Razporeditev ustnač v zalivu Duče med posameznimi urami vzorčenja.
In total, more sampled Labrids were found to be diurnal fishes. That fact is probably connected with their usage of algae for feeding and shelter during sunlight. Wrasses are strongly diurnal (only active during the daytime) and, like parrotfishes, many bury themselves in the sand or seek crevices to hide in at night (Choat & Ayling, 1987). S. ocellatus and S. roissali were mainly diurnal, while at night more S. cinereus specimens were found. The night occurrence could be explained as either an increased catchability at night or a movement of these individuals into the analysed area at dusk and leaving at dawn or some combination of both (Nash et al., 1994). However, it is very speculative to pronounce any of those species as diurnal or nocturnal, as only with the
non-destructive method sampling we would be able to assess the real diel aspects of certain species. The highest catches were recorded by sampling in 12 h (82 individuals) and 24 h (59 individuals) periods. However, there was no significant difference in Labrid catches between day and night samples, indicating that one year of investigation is probably not enough or that more frequent sampling is needed. It is well known that the behaviour connected with the day/night light cycle differs between species and that there is often a difference in catchability or vulnerability of species relative to that point (Parsley et a/., 1989). In addition, individuals do not see nets at such great distances during night, and therefore capture efficiencies are higher at night. In the Mediterranean P. ocean/ca beds, Harmelin-Vivien (1982) attributed the increased abundance and diversity of the fish fauna at night not only to the immigration of nocturnal macrophagic carnivores from the adjacent reefs, but also to the movements of diurnal planktivores from the water column to the sheltered sites beneath the canopy.
It is apparent from the foregoing discussion that a number of parameters influence the differential distribution of Labrids, especially the occurrence and composition of their juveniles. Their occurrence in the Duce area is surely the result of juvenile preference for relatively shallow and nutrient rich coastal water. Also, those areas provide shelter from possible predators, whose manoeuvrability may be hampered in shallow water (Dul-cic et a/., 1997).
Fig. 6: Non-Metric Multidimensional Scaling (nMDS) ordination plot for Labrids among day and night samples (stress < 0.01): A day, Z night. Sl. 6: Nemetrično multidimenzionalno skaliranje (nMDS) za ustnače med dnevnimi in nočnimi vzorci (pomen < 0,01): A dan, Z noč.
The results of the present study provide a basis for establishing the temporal and spatial patterns of occurrence and recruitment of the Adriatic Labrids in coastal area. It is necessary to establish their monitoring due their tropic origin, increasing abundance of the present Labrids as well as their movement to the northern parts of the Adriatic. A further research is also needed due to a reasonable expectation of new Labrid species occurring as a consequence of climate change.
SEZONSKE SPREMEMBE V DNEVNEM ULOVU USTNAC V PLITVINAH ZALIVA DUCE GLAVA V VZHODNEM JADRANU
Sany'a M4T/Č-S.O.O, -aNov D8LC/Č, 0/ro .RALjEV/Č & L/v//a TOM4SOV7Č Inštitut za oceanografijo in ribištvo, HR-21000 Split, P.O. BOX 500, Hrvaška E-mail: sanja@izor.hr
POVZETE.
$vforj/ pr/spevNa so med apr//om 2000 /n marcem 200? vzore/// združbo p//fNovodn/K r/b, ujef/K z manjšo mrežo vzdo/ž peščene oba/e Duce G/ava južno od Sp//fa (vzKodn/ jadranj. Z namenom, da b/ preuč/// raz//Ne v 24-urnem obdobju v pr/merjav/ s sfruNfuro združbe v eno/efnem obdobju, so mesečne zb/rNe vzorcev razde//// na f/sfe, u/ov/jene podnev/, /n f/sfe, u/ov/jene ponoč/. 0ed ce/ofn/m preučevan/m obdobjem je b//o vzorčen/K 284 usfnač (z mnog/m/ m/adosfn/m/ osebN/ med nj/m/j, pr/padajoč/K 6 raz//čn/K vrsfam (Symphodus ocellatus, S. cinereus, S. rois-
sali, S. rostratus ter Labrus viridis in L. merula). Pri vsaki od teh ustnač je bil ugotovljen drugačen vzorec njihove dnevne in sezonske številčnosti. S. ocellatus je bila dominantna vrsta prek celega leta. Največji prirastki v številu vseh ustnač iz rodu Symphodus so bili zabeleženi v poletnem obdobju. Vrsta L. viridis, ki so jo zastopali največji osebki med vsemi vzorčenimi ustnačami, pa v poletnem in jesenskem obdobju sploh ni bila zabeležena. Medtem ko so bile S. ocellatus, S. roissali in S. rostratus videti predvsem dnevne ribe, je bilo ponoči ujetih največ osebkov vrst S. cinereus in L. viridis.
Ključne besede: sezonskost, dnevno-nočne razlike, ustnače, Jadransko morje, obalno območje
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