ECOLOGICAL TRAITS OF THE PATAGONIAN BLENNY, ELEGINOPS MACLOVINUS (PISCES, ELEGINOPIDAE), IN THE RÍO GALLEGOS ESTUARY (SOUTHWESTERN ATLANTIC)

Communitary and population analysis of Eleginops maclovinus

A total of 788 fish individuals were counted during the study period. The specimens collected included 9 identified species (Table I), reaching the highest values in February (7 spp.) and the lowest in May (1 sp.), remaining below 4 spp. during the rest of the months sampled. The patagonian blenny Eleginops maclovinus was the main species caught (ca. 81 % of total specimens collected), followed by the silversides Odontesthes smitti (ca. 10 %) and Odontesthes nigricans (ca. 6 %), the brown trout Salmo trutta (1 %), while fueguian sprat Sprattus fuegensis, butterfish Stromateus brasiliensis, cod icefish Patagonotothen canina, parona leatherjack Parona signata and spiny dogfish Squalus acanthias registered the lowest abundances (< 1 %). In addition, of the total fish caught on a monthly basis, E. maclovinus was the only species recorded throughout the entire study period and displayed the highest values (ca. 20-100 individuals per month), followed by O. nigricans and O. smitti (ca. 1-50 individuals per month), whereas the rest of species were observed occasionally (1-4 individuals per month).

E. maclovinus specimens showed a size range between 480-520 mm. The abundance of E. maclovianus showed a significant difference between months and tides (in both cases two-way ANOVA, P < 0.001; Table II, Fig. 2B). The mean values of E. maclovinus caught varied between 4 and 7 individuals per day from December to April, reaching the lowest values in May (1 individual per day; SNK test, P < 0.05). The comparison of tidal regimes established that E. maclovinus registered higher abundances during night (ca. 4 individuals per day) than daytime tide (2 individuals per day; SNK test, P < 0.05). Spearman correlation coefficient did not show a significant relationship between seawater temperature (r = 0.37; P > 0.05) and photoperiod (day duration vs daytime tide abundance, r = 0.49; night duration vs night tide abundance, r = -0.77; in both cases P > 0.05) with E. maclovinus specimens caught.

Feeding analysis of Eleginops maclovinus

Feeding analysis revealed that from the total specimens of E. maclovinus (n = 448), 92.9 % had contents in the stomach while 7.1 % were empty (Fig. 3A). A total of 11 prey items were recorded (Table III), varying monthly between 3 to 9 spp. in January and April, respectively. The composition was represented by crustaceans (5 spp.), fish (3 spp.), algae (2 spp.) and molluscs (1 sp.). The most abundant species in the stomach contents were the bivalve Darina solenoides (ca. 76 %; Fig. 3B), followed by the decapod Grimothea gregaria (ca. 5 %; Fig. 3C). The peracarids Acanthoserolis schythei (Fig. 3D), Edotia tuberculata, Mysid sp., the decapod Peisos petrunkevitchi, the osteichtyes Odontesthes smitti, Patagonotothen canina and Sprattus fuegensis, and the algae Phaeophyceae sp. and Ulva sp., accounted for less than 2 % of the stomach contents in average.

The taxon with the highest mean % FO in the stomach contents was D. solenoides (71.37 %), while the rest of species registered mean values lower than 7 %. Monthly % FO showed that D. solenoides was the dominant prey species with abundance percentage during all the months sampled, ranging between ca. 55 % (April) to ca. 83.82 % (March; Fig. 4); followed in January by P. petrunkevitchi (ca. 11 %), G. gregaria and S. fuegensis (ca. 6-8 %); in February by Ulva sp. (ca. 9 %) and G. gregaria (ca. 6 %); in March by G. gregaria (ca. 12 %) and Ulva sp. (ca. 9 %); in April by A. schythei (ca. 15 %), G. gregaria, Ulva sp. and P. canina (in all cases, ca. 7-9 %); and in May by P. canina, Phaeophyceae sp. and Mysida sp. (in all cases, ca. 9 %); while during the rest of the months sampled species registered a % FO lower than 5 %.

DISCUSSION

Previous studies have shown that Río Gallegos estuary shows a high species richness of fishes with a total of 35 species recorded (Caille et al. 1995, Torres et al. 2006, Torres 2009), of which 12 were classified as resident (i.e. species present throughout the year), 11 as seasonal (i.e. species recorded in a particular season) and 12 as occasional (i.e. species not recorded on consecutive occasions; Torres et al. 2006, Torres 2009). The present study found that from the total number of fish species in the estuary, only nine were registered in the intertidal zone during high tide, six of which were resident species (i.e., Odontesthes nigricans, Odontesthes smitti, Sprattus fuegensis, Eleginops maclovinus, Patagonotothen canina, and Squalus acanthias), while the remaining three were seasonal (i.e., Parona signata, Stromateus brasiliensis, and Salmo trutta). Resident fishes use the estuary as trophic and/ or breeding sites, explaining their highest abundance in the gill nets (Caille et al. 1995, Torres et al. 2006). However, some of these species were recorded occasionally (e.g., S. fuegensis, P. canina, and S. acanthias). This could be due to several factors, such as their distribution being restricted to the estuary mouth or the subtidal zone, their abundance is in decline or more gill nets are needed to increase the sampling effort (Torres 2009). On the other hand, the presence of seasonal species has been linked to their migratory and reproductive behavior. For example, species such as P. signata and S. brasiliensis extend their distribution from northern to southern latitudes in summer, affected by a warmer current (i.e. Brazilian current), entering in the estuary to feed (Torres 2009, Cousseau & Perrotta 2013). Meanwhile, species such as S. trutta move from the sea to the estuary for reproduction (Torres 2009, Cousseau & Perrotta 2013). However, to determine the population status of resident and temporal species in the estuary and explain the low abundances found in the present study, additional information is required. The monthly abundance variation of E. maclovinus may be attributed to its reproductive cycle.

Brickle et al. (2005) reported that the spawning period of this species from the Malvinas (= Falkland) Islands population occurs in deeper nearshore waters (> 40 m) during austral spring and secondarily in autumn, entering estuaries only to feed. Thus, the fact that adult individuals of E. maclovinus showed a total length close to the optimal size for sex-changing (360-500 mm; see Brickle et al. 2005, Licandeo et al. 2006), and that a high abundances of specimens were recorded from spring to summer in the inner part of the estuary (ca. 30 km from the mouth) supports the idea that the intrusion of E. maclovinusto the Río Gallegos estuary was mainly related to their foraging activity, obtaining nutrients and energy for their growth and gonadal maturation (Zama & Cárdenas 1982, Brickle et al. 2005, Licandeo et al. 2006, Di Giácomo et al. 2015). Reversely, the decrease in its abundance during autumn could be related to a migration process from shallow to deep coastal areas for reproduction.

Fish catchability may increase in response to environmental factors such as water temperature and photoperiod duration (Hammerschlag 2017; Leitão et al. 2018). However, no relationship was observed between these two parameters and the number of E. maclovinus caught. On the other hand, a significant difference in E. maclovinus abundance was recorded between daytime and night tides. It has been observed that fish species adopt different strategies to take advantage of light or dark conditions (Hammerschlag et al. 2017). Some species are more active at night to reduce their vulnerability to predators (Grol et al. 2008, Dupuch et al. 2009, Hammerschlag et al. 2010, 2017). Previous studies have shown that benthic macroinvertebrate abundances in the intertidal zone of Río Gallegos estuary is higher than in the subtidal zone (Pittaluga 2016, Lizarralde et al. 2017, 2019). Thus, the highest abundance of E. maclovinus during night tides could be due to this species moving from the subtidal to the intertidal zone to feed more safely. However, this hypothesis requires further evaluation, including factors such as differences in net visibility and moon phase effects (Hammerschlag et al. 2017). In general, the fish species collected during the study period mainly feed on plankton and fish (Torres 2009, Cousseau & Perrotta 2013). E. maclovinus is an exception as it has been classified as a predator of benthic species (Pequeño 1979, Licandeo et al. 2006, Martin & Bastida 2008, among others). The stomach contents of E. maclovinus from Rio Gallegos population also consisted solely of benthic organisms. The species composition was low (11 spp.) compared to other populations from coastal and estuarine areas of southern Patagonia (ca. 27-91 spp.) (Pequeño 1979, Isla & San Román 1995, Licandeo et al. 2006, Martin & Bastida 2008, Gastaldi & Gonzaléz 2009). Although, some populations from southern Pacific Patagonia registered similar values (ca. 11-14 spp.; Zama & Cárdenas 1982, Pavés et al. 2005, Pequeño et al. 2010). The low abundance of food items may be related to the lowest benthic species richness recorded in the Río Gallegos estuary (ca. 30 spp.), compared to nearby estuarine and marine populations, in which a highest abundance of food items and benthic species richness was recorded (32 and 69 spp. respectively; Martin & Bastida 2008, Lizarralde et al. 2017, Martin et al. 2019). However, it should be noted that studies where more food items were recorded, covered longer sampling periods (between 1-3 years; i.e. Pequeño 1979, Isla & San Román 1995, Licandeo et al. 2006, Martin & Bastida 2008, Gastaldi & Gonzaléz 2009). Therefore, it cannot be excluded that the lowest food items registered in Río Gallegos estuary could be related to the shorter sampling period, restricted only to the fishing season (ca. 6 months; Di Giácomo et al. 2015). Throughout the sampling period, the bivalve D. solenoides dominated the stomach contents of E. maclovinus in terms of both percentage volume and frequency of occurrence. These results suggest that D. solenoides plays a crucial role as an energy source for E. maclovinus, (Lizarralde & Pittaluga 2011, Lizarralde et al. 2018). D. solenoides is the dominant species in the middle intertidal level, with consistent density and biomass through the year (Lizarralde & Pittaluga 2011, Lizarralde et al. 2018, Gallardo et al. 2012, Martin et al. 2019). These results indicate that E. maclovinus was dependent on a specific prey, unlike other populations for which a wider prey item were found (in terms of quantity and occurrence) (Guzmán & Campodónico 1973, Pequeño 1979, Pavés et al. 2005, Licandeo et al. 2006, Martin & Bastida 2008, among others). Additionally, the presence of other food items in E. maclovinus specimens, such as crustaceans (i.e. Acanthoserolis schythei, Edotia tuberculata, Mysida sp., Grimothea gregaria, Peisos petrunkevitchi) and fish (Odontesthes smitti, Patagonotothen canina and Sprattus fuegensis), indicates that the feeding habits of E. maclovinus appears to be generalist and opportunistic, feeding mainly on those resources that are found in greatest abundance (Guzmán & Campodónico 1973, Pequeño 1979, Pavés et al. 2005, Licandeo et al. 2006, Martin & Bastida 2008).

In summary, this study concludes that E. maclovinus is the dominant species in the intertidal zone. The comparison of fish abundance between daytime and night tides suggests that this species is more active at night, possibly to avoid predation risk. The stomach contents revealed that E. maclovinus is an opportunistic species, feeding on most abundant and available benthic organisms. The present study will provide important information to design biological and ecological studies tending to determine the status of natural resources in the Rio Gallegos estuary, in order to develop appropriate legislation for the implementation of sustainable artisanal fisheries.