Ontogenetic and seasonal shifts in the diet of the pemecou sea catfish Sciades herzbergii (Siluriformes: Ariidae), from a macrotidal mangrove creek in the Curuçá estuary, Northern Brazil

Ontogenetic and seasonal shifts in the diet of the pemecou sea catfish Sciades herzbergii (Siluriformes: Ariidae), from a macrotidal mangrove creek in the Curuçá estuary, Northern Brazil Tommaso Giarrizzo1,2 & Ulrich Saint-Paul2 1. 2. Laboratório de Biologia Pesqueira - Manejo dos Recursos Aquáticos, Universidade Federal do Pará (UFPA), Av. Perimetral 2651, Terra Firme, 66040170 Belém, PA - Brazil; Center for Tropical Marine Ecology (ZMT), Fahrenheitstr. 6, 28359 Bremen, Germany; Received 11-iv-2007. Corrected 26-X-2007. Accepted 09-iii-2007. Abstract: The feeding ecology of pemecou sea catfish (Sciades herzbergii) was studied bimonthly from September 2003 to July 2004 in a macrotidal mangrove creek at Curuçá estuary, Brazil. Feeding activity and relative importance of prey groups were assessed in 528 and 226 specimens (6.2 to 36.0 cm total length), respectively, using stomach contents. A total of 1 820 specimens were caught. The fish left the creek with an average of 2.2 % of their body weight in prey. Feeding activity was lower in the dry season (September and November 2003) than in the rainy season (January to May 2004). The diet was dominated by Ocypodidae and Grapsidae. Feeding strategies of the pemecou sea catfish indicated a slight variation between the diets of adults and juveniles; however; the main factor leading to diversification in the diet were the seasons. Opportunistic feeding behavior was observed in the dry season, shifting to more specialized feeding in the wet season. This temporal diet shift may be explained by the environmental seasonal variations (e.g. transparency of the water, high water level) in the intertidal mangrove creek habitat. Rev. Biol. Trop. 56 (2): 861-873. Epub 2008 June 30. Keywords: Sciades herzbergii, feeding strategy, Ariidae, mangrove, stomach contents, Ocypodidae, Grapsidae. Ariids are medium to larger-sized demersal catfishes, mostly inhabiting estuarine and marine environments in tropical and sub-tropical regions (Acero and Betancur 2007), and are an important component of many tropical fisheries (Darracott 1977, Tobor 1978, Vasudevappa and James 1980, Conand et al. 1995). The pemecou sea catfish, Sciades herzbergii (Bloch 1794), is distributed along the Atlantic coast from the Caribbean coast of Colombia to Northeast Brazilian coast (Sergipe) (Oliveira 1976, Froese and Pauly 2007). The species inhabits the brackish turbid waters of estuaries (Le Bail et al. 2000). On the Northern coast of Brazil, S. herzbergii is an abundant species in the intertidal mangrove habitat (Barletta et al. 2003, Krumme et al. 2004, Giarrizzo and Krumme 2007a) and is an important resource both for the subsistence and the economy of the local population. Usually the artisanal fishermen capture the pemecou sea catfish in first order intertidal mangrove creeks using non-selective fishing methods such as block nets (mesh size 3 cm) or fish trappings, thereby causing an adverse effect on fish recruitment (Giarrizzo and Krumme 2007b). Although pemecou sea catfish is an important fishery resource along the Brazilian coast, information on its biology and ecology is still lacking. The purpose of the present study is to describe the feeding ecology of pemecou sea catfish, by examining the ontogenetic and seasonal shift in diet in a macrotidal mangrove creek in the Curuçá estuary. Rev. Biol. Trop. (Int. J. Trop. Biol. ISSN-0034-7744) Vol. 56 (2): 861-873, June 2008 861 MATERIALS AND METHODS Study area: The present study was carried out in a mangrove tidal creek in the Curuçá estuary, approximately 160 km northeast of Belém, Pará, Northern Brazil (0°10´ S, 47°50` W). This estuary has been designated as a conservation unit and is relatively undisturbed. The area is characterized by well-developed mangrove forests dominated by Rhizophora mangle (L.), Avicennia germinans (L.), and fewer Laguncularia racemosa (L.). Average precipitation on the estuary is 2 526 mm (ANA 2006; n= 16 years, range: 1 085–3 647 mm). Tides are semidiurnal and tidal amplitude in the estuary is about 4 m. The whole mangrove swamp is inundated at high tide for only a few hours during spring tide. Samples were collected in a first order tidal creek located in the upper estuary at 1.2 km from Curuçá city. The perimeter, length and area of this creek is 777 m, 277 m and 20 000 m2 respectively. The surface sediment of the inundated area is composed mostly of clay and silt, but there is also bedrock in the mouth of the creek. Sampling: Samples were taken bimonthly between September 2003 and July 2004 from a tidal creek during the first quarter lunar phase, using a fyke net. This fishing gear is a passive sampler composed of two wings (20 m long, 6 m deep, with 20 mm stretch-mesh) and a hoopnet (with 13 mm stretch-mesh). The fyke net was set at the mouth of the creek at daytime slack high water (HW). In order to limit the regurgitation and digestion rates, during the tidal reflux (6 hours on average), every 15 minutes the fyke net was lifted to collect fish. For each sampling event, salinity (salinity refractometer) and water transparency (Secchi disc) were recorded every 30 minutes in the mouth of the intertidal creek during ebb tide. The high water level was measured with a ruled stake placed in the thalweg of the creek downstream close to the net. The topography of the creek was surveyed (GPS, compass, tape measure) and a GIS map was generated. 862 The inundated area (m2) for a medium neap tide event was quantified for the creek using ArcGIS. Thus, catch weights and abundance were standardized to biomass (g m-2) and density (ind. m-2), respectively. Diet analysis: All specimens were measured to the nearest 0.1 cm total length (TL), and weighed to the nearest 0.01 g wet body weight (BW). The catalogue number of voucher specimens is MPEG12653 (Museu Paraense Emilio Goeldi, Belém, Pará, Brazil). At least 60 individuals were randomly taken from the total bimonthly catch to assess feeding activity. Subsequently, a minimum of 20 specimens with food in the stomach were picked for stomach content analysis. After dissection, the stomach contents of each fish were weighed to the nearest 0.01 g (stomach content fresh weight, FW). To measure which proportion of the fish weight was actual food consumed, the fullness index (%FI) was calculated following the formula defined by Hynes (1950): %FI = (FW / BW) × 100 The number of empty stomachs was counted and the vacuity index (%VI) was calculated: %VI = (Total of empty stomachs / Total amount of stomachs analyzed) × 100 As an indicator of energy storage, the Fulton’s condition factor (K) of each specimen (n= 528) was calculated from the relationship: K = (EW × TL-3) × 100 where EW is the eviscerated wet body weight (g). The assumption of isometric growth with the constant equal 3 (Bolger and Connolly 1989) was examined for the 528 specimens and no significant difference was detected with the allometric coefficient (b) of length-weight relatio (...truncated)