PEDRO MIGUEL ALFAIA BARCIA RÉ

ICTIOPLÂNCTON DA REGIÃO CENTRAL DA COSTA PORTUGUESA E DO ESTUÁRIO DO TEJO

ECOLOGIA DA POSTURA E DA FASE PLANCTÓNICA DE SARDINA PILCHARDUS (WALBAUM, 1792) E DE ENGRAULIS ENCRASICOLUS (LINNÉ, 1758)

LISBOA

1984

ABSTRACT

Very few studies have been made in Portugal on marine fish eggs and larvae. The present work deals with: (i) the diversity and abundance of the ichthyoplankton along the central portion of the portuguese coast and within Tejo estuary; (ii) The ecology of the spawning and of the planktonic phase of Sardina pilchardus (Walbaum, 1792); (iii) The ecology of the spawning and of the planktonic phase of Engraulis encrasicolus (Linné, 1758), within Tejo estuary. Fish eggs and larvae were sampled over a period of two years off the central part of the Portuguese coast and over a period of four years within Tejo estuary. The diversity and abundance of the ichthyoplankton was analysed and related with upwelling conditions and plankton biomass. Ichthyoplankton diversity was highest in spring and summer and lower in winter, along the central part of the Portuguese coast. Off the coast of Sines (1981/1982) the diversity of the ichthyoplankton was probably influenced by upwelling conditions. Within Tejo estuary the diversity was, with few exceptions, highest at exterior stations and lower at interior stations during the four year survey (1978/1981). A significant correlation was found to exist between plankton biomass and the number of larvae sampled per month at each station off the coast of Peniche (1979/1980). Within Tejo estuary the ichthyoplankton was nearly always more abundant at interior stations. Sardina pilchardus eggs and larval stages were the most abundant forms sampled along the central part of the Portuguese coast. The analyses of the Euclidean distance cladograms of collected fish larval stages, showed that off the central part of the Portuguese coast the stations sampled were not very dissimilar and that there was not a definite gradient of ichthyoplankton abundance. Within Tejo estuary interior stations were normally very dissimilar in relation to the lower estuary. It was included under the designations of winter spawners, spring spawners, summer spawners and autumn spawners the species whose eggs and larvae were captured along the central part of the Portuguese coast during these seasons. Within Tejo estuary it was possible to distinguish between the species that use the estuary as a primary spawning area from those that do not. The eggs of Sardina pilchardus were sampled throughout a period of twelve months along the inshore waters of the central portion of the Portuguese coast. Peak periods were found in December and April (1979/1980) off the coast of Peniche and in March and June (1981/1982) off the coast of Sines. Spawning was temperature dependent. Eggs were sampled in greatest numbers when water temperatures were not higher than 15‑16ºC. Off the coast of Sines the upwelling conditions influenced both spawning activity and duration. The existence of two spawning peaks off the coast of Peniche in 1979/1980 was related to the age structure of the spawners. The size and relative mortality of pilchard eggs varied seasonally. The time of spawning was deduced from the presence of early stage eggs in the plankton. Sardina pilchardus seems to spawn during the first hours of darkness. Pilchard larvae showed a clear diel vertical migration being found predominantly at a daytime depht somewhere near 20 meters and at the surface during the night. Larvae also showed a cycle of feeding activity, suggesting that feeding took place mainly if not exclusively during daytime. Sardine larvae were aged using daily growth increments in sagittal otoliths. Growth and thickness of daily units were found to varie in relation to the time of the year. This variation was related to differences in food availability and different feeding conditions. No obvious relationship was found with temperature. The hour of the formation of the two parts of each daily unit (organic and inorganic) was established. The protein‑rich portion of the daily increment is deposited during the first hours of darkness. The initiation of a new daily growth increment begins around midnight. The number and biomass of Sardina pilchardus spawners was estimated from quantitative egg surveys: coast of Peniche (1979/1980) (6,70.10³ Ton); coast of Sines (1981/1982) (1,33.10³ Ton). The eggs and larval stages of Engraulis encrasicolus were sampled troughout a period of six months (February to July) during four years (1978/1981) within Tejo estuary. Spawning activity was highest in April and took place with greater intensity at the interior portions of the estuary. The fact that anchovy larvae were present in greater concentrations in the upper portions of Tejo estuary, and that larvae of higher length‑classes were always found at interior stations, led us to believe that there must be some mechanism responsible for its retention within the estuary. Egg mortality increased and its dimensions decreased as the spawning season progressed. Anchovy showed a crepuscular spawning; early stage eggs were found between 18:00 and 00:30h, local time. Retention of anchovy larvae within Tejo estuary was also studied. This retention was mainly associated with the location of the spawning grounds and coupled with the average conditions of water circulation inside the estuary.