Investigation into the early life history of the European flounder (Platichthys flesus L.) with special emphasis in Galway Bay, Irish West Coast

Abstract

European flounder (Platichthys flesus L.) economic value is estimated to be worth over 68 million to the Irish economy (mainly from recreational angling activities). Despite such large species value, there is a considerable lack of data for this region, particularly concerning stock structure, timing of life history events and juvenile habitat use patterns. This dissertation addresses these knowledge gaps and describes a number of studies which focus on the understanding of flounder nursery ground ecology. Understanding stock structure and the connectivity between geographically distinct groups of fish is critical for the sustainable management of a species. Meristic analysis revealed a modest separation of juvenile flounder from different regions (west coast of Ireland, east coast of Ireland and the Welsh coast). The variation between regions was subtle and there was a large degree of overlap. The technique is therefore not a powerful method of stock identification on its own but may improve the likelihood of detecting flounder stock structure if combined with other markers in a multidisciplinary approach to stock identification. Little is known of the early life traits of flounder and the affect early benthic life has on habitat use patterns and survival. Otolith microstructure analysis established critical baseline data on the timing and duration of early life history events of juvenile flounder. Peak hatching and settlement occurred in February/March and March/April respectively while the average pelagic larval phase was on average 43 days. Spatial and temporal variability in early life history traits and size were identified which may be related to inter/intra estuarine movement of flounder and/or selective mortality of different settlement cohorts. Flounder sampled from beaches and estuaries within Galway Bay showed different habitat use patterns. Individuals on the beaches used the habitat over a short period of time and were either moving away from the beach habitat or undergoing mortality subsequent to arrival; whereas flounder were continuously found in the estuaries until late summer. There is evidence that biochemical (RNA:DNA) and morphometric condition represent different time scales in fishes’ life which can be useful for studying short and long term condition of fish. Under experimental laboratory conditions, post larval flounder held at salinity of 30 had higher RNA: DNA compared to individuals held at salinity of 0 while no difference in morphometric condition was detected. In contrast, in the estuarine environment flounder condition (morphometric and RNArDNA) increased as salinity decreased. The findings of this thesis provided important information on the dynamics of 0-group flounder which can aid conservation of essential juvenile habitats which in turn can positively affect recruitment of 0-group individuals to the adult spawning population.