Flatfish Abundance Research Articles

On the factors influencing juvenile flatfish abundance in the lower Severn Estuary, England

Bridgwater Bay within the Bristol Channel, Somerset, England is a nursery ground for sole, Solea solea, and dab, Limanda limanda, during the autumn and winter. Flounder, Platichthys flesus, both juveniles and adults, are common during the summer. Using a 13-year data set of fish in the bay, correlations were studied between climatic, predatory and competitive factors and juvenile flatfish abundance. The major factor was found to be seawater temperature. For sole, abundance was positively correlated with the temperature in the spawning period (April and May). For flounder, abundance was negatively correlated with average temperature during the previous year. For dab, average winter temperature over the spawning period was negatively correlated with juvenile abundance and with mean length observed during the following autumn. These climatic changes were also found to influence the abundance of a large number of other fish and crustacean species which were potential predators or competitors and which in some cases were significantly correlated with flatfish abundance. The data set was analysed using multiple correlation analysis. Multi-factorial models of population change which included interspecific and climatic factors were examined. Using first-order partial correlations it was possible to distinguish between different causal models. In every case it was found that inter-specific correlations were attributable to both species independently changing in abundance with temperature. No significant correlations between the abundance of potential predators or competitors and juvenile flatfish were detected.

Settlement, abundance, growth and mortality of juvenile flatfish in a subtropical tidal estuary (Georgia, U.S.A.)

The population dynamics of juveniles of some flatfish species were studied in the Duplin River, a tidal creek in a subtropical salt-marsh area in Georgia, U.S.A. from April until September 1990. Seven species were found. Paralichthys dentatus, Paralichthys lethostigma, Paralichthys oblongus and Trinectus maculatus were relatively rare. Etropus crossotus, Citharichthys spilopterus and Symphurus plagiusa were abundant and settled during the period studied. E. crossotus was the most abundant species with a mean abundance of 18 ind·10 −2 m −2 (max. 287). Demersal settlement of E. crossotus took place in shallow areas and over sandy bottoms from mid-May to August. Prolonged settlement hampered the calculation of growth rate and instantaneous mortality rate. However, laboratory growth experiments indicated a mean growth of about 0.50 mm·d −1 at 24–28°C. Juveniles of C. spilopterus were already present in the Duplin River in March. Settling continued until the end of April with a mean abundance of 3.5 ind·10 −2 m −2 (max. 183). With increasing size the juveniles of this species tended to migrate to deeper waters and to the mouth of the river, possibly as a reaction to increasing water temperatures. Maximum growth rate was 1.4 mm·d −1 at about 26°C. The mean instantaneous mortality rate (z) was estimated at 0.03·d −1. Settling of S. plagiusa occurred from mid-May onwards. The mean abundance was 10.3 ind·10 −2 m −2 (max. 98.3). Newly settled juveniles were most abundant on muddy sediments in the shallow river areas. The maximum growth rate was 1.3 mm·d −1 at about 28°C. The mean instantaneous mortality rate (Z) decreased from 0.04·d −1 in April to 0.01·d −1 in August. At all sites the abundance of juveniles of this species decreased with increasing water depth. Predation experiments indicated that blue crabs ( Callinectes similis and C. sapidus) and sea robins ( Prionotus sp.) are potential predators on juvenile flatfish. The high abundances of juvenile flatfish indicate that the tidal creeks are an important nursery area. The correspondence between growth rates estimated from field data and those observed in the laboratory suggests that growth in the nursery is mainly related to water temperature and not food limited.

Abundance, biomass and production of juvenile flatfish in southeastern kattegat

Abundance, biomass and production of juvenile 0- and 1-group flatfish were estimated at 1.5 to 11.0 m depth from May 1984 to May 1987 in southeastern Kattegat. Species studied were: Plaice, Pleuronectes platessa (L.), sole, Solea solea (L.), dab, Limanda limanda (L.), turbot, Scophthalmus maximus (L.), brill, Scophthalmus rhombus (L.), and flounder, Platichthys flesus (L.). Highest abundance and biomass of 0- and 1-group flatfish occurred in July and August each year. Plaice, sole, turbot, brill and flounder were mainly found as 0-group at 1.5 to 5.0 m, but as 1-group they also occupied deeper water. 0- and 1-group dab occurred in the highest density at 5.0 to 11.0 m. Total summer (May to September) production at 1.5 to 5.0 m of the dominant species, plaice, sole and dab, were 98, 23 and 88 g AFDW per 100 m 2 during the three years investigated. Corresponding figures for the depth range 5.0 to 11.0 m were 12, 13 and 53 g AFDW per 100 m 2. Effects of eutrophication on the area as a nursery ground for flatfish are discussed.

NORTH ALEUTIAN SHELF SEA OTTERS AND THEIR VULNERABILITY TO OIL

ABSTRACT Results of a one-year field investigation on the sea otters of the eastern Bering Sea indicate that this population ranges from approximately 1,000 during the winter to over 10,000 in the summer, apparently due to migration from the Pacific. Even though individuals are known to occur over 20 mi offshore in 40 m of water, otters were found primarily in shallow waters (0 to 20 m) off the major lagoons and bays. These seasonal and spatial distributions correspond better with the abundance of important prey, flatfish and crabs, rather than with other environmental factors. The otters are believed to feed on a variety of prey, including crabs, molluscs, flatfish, and perhaps sand dollars, based on scat analysis. Significant impacts from oil and gas activities on this population would occur if a large spill occurred near Bechevin Bay, along the apparent major migration route. Other impacts would be more localized and are difficult to estimate due to the need for information on the otters' ability to avoid oil slicks and at the same time maintain their feeding regimes.