Richard D.M. Nash

Length–weight relationships for 21 coastal fish species of the Azores, north-eastern Atlantic

Abstract Length–weight relationships were estimated for 21 coastal fish species of the Azores, namely Abudefduf luridus, Bothus podas, Chromis limbata, Coris julis, Diplodus sargus, Echiichthys vipera, Gaidropsarus guttatus, Labrus bergylta, Mullus surmuletus, Phycis phycis, Pomatomus saltator, Sarda sarda, Scorpaena maderensis, Scorpaena notata, Seriola rivoliana, Serranus atricauda, Sparisoma cretense, Sphyraena viridensis, Synodus saurus, Thalassoma pavo and Trachinotus ovatus . Significant length–weight relationships were found for all species. Sexual dimorphism did not affect the length–weight relationships, except in the cases of S. cretense and C. julis . Length–length equations for converting size measurements (standard length (SL) and fork length (FL) to total length (TL)) are also presented for all fish species.

Synergies between climate and management for Atlantic cod fisheries at high latitudes

Significance Currently many exploited fish populations, including many of the Atlantic cod stocks, are at historically low levels with widespread concern about whether contemporary management is capable of facilitating population recovery. In contrast, the spawning stock biomass of Barents Sea cod is now at an historic high. Here we demonstrate that successful management actions interacting synergistically with prevailing climate caused this increase. Warming of water masses in the Barents Sea over the last decade positively reinforced management actions. A unique and possibly generic mechanism of climate affecting marine animals at high latitudes, especially when at the polar extreme of their distribution, is identified: adjustment of the suitable feeding area. This adjustment is linked closely to community dynamics and increased stock productivity. The widespread depletion of commercially exploited marine living resources is often seen as a general failure of management and results in criticism of contemporary management procedures. When populations show dramatic and positive changes in population size, this invariably leads to questions about whether favorable climatic conditions or good management (or both) were responsible. The Barents Sea cod (Gadus morhua) stock has recently increased markedly and the spawning stock biomass is now at an unprecedented high. We identify the crucial social and environmental factors that made this unique growth possible. The relationship between vital rates of Barents Sea cod stock productivity (recruitment, growth, and mortality) and environment is investigated, followed by simulations of population size under different management scenarios. We show that the recent sustained reduction in fishing mortality, facilitated by the implementation of a “harvest control rule,” was essential to the increase in population size. Simulations show that a drastic reduction in fishing mortality has resulted in a doubling of the total population biomass compared with that expected under the former management regime. However, management alone was not solely responsible. We document that prevailing climate, operating through several mechanistic links, positively reinforced management actions. Heightened temperature resulted in an increase in the extent of the suitable feeding area for Barents Sea cod, likely offering a release from density-dependent effects (for example, food competition and cannibalism) through prolonged overlap with prey and improved adult stock productivity. Management and climate may thus interact to give a positive outlook for exploited high-latitude marine resources.

The influence of nursery ground processes in the determination of year-class strength in juvenile plaice Pleuronectes platessa L. in Port Erin Bay, Irish Sea

The interannual variability in settlement and mortality of juvenile plaice Pleuronectes platessa L. was investigated between 1992 and 1998 on Port Erin Bay, west side of the Isle of Man, Irish Sea. The dampening influence of factors operating on the nursery grounds was especially obvious in 1992 and 1996. In these years extremely high numbers of individuals settled, yet the population sizes in July were similar to other years. Thus the nursery ground processes were likely to be density dependent. Shrimp and crab densities were low in Port Erin Bay and probably had little predatory impact on young plaice. Crustacean densities were not significantly related to winter temperatures. In the Irish Sea, year-class strength is determined during the nursery ground phase, in contrast to the North Sea where determination of year-class strength occurs prior to the nursery ground phase.

Reproductive cycle and energetic cost of reproduction of the neogastropod Buccinum undatum in the Irish sea

The annual reproductive cycle and cost of reproduction of the common whelk, Buccinum undatum L. (Gastropoda: Prosobranchia) were studied off Douglas, south-east of the Isle of Man. Sexual maturity was reached at 60–70 mm shell length. Two years of laboratory observation and field data indicated an autumn to mid-winter breeding season. Egg-laying occurred in the laboratory mainly between December and January, and hatching of juveniles between April and early May, 3–5 months after spawning. The cycles of testis and ovary development were very similar, especially during the breeding period. The annual energy invested in reproduction was estimated to be at least 4.1 kj m -2 . The investment by females (3.84 kj m -2 y -1 ) was much greater than by males (0.26 kj m -2 y -1 ).

The concept of fecundity regulation in plaice (Pleuronectes platessa) tested on three Irish Sea spawning populations

The fecundity of European plaice (Pleuronectes platessa) in the Irish Sea between 2000 and 2004 was estimated during the spawning season for fish in the three main spawning areas (Liverpool Bay, the Cumbrian coast, and the western Irish Sea) and one small spawning group on the west coast of the Isle of Man. Fecundity was also estimated during September of 2003 and 2004. The aim of this was to assess the variability in fecundity between areas and years in the Irish Sea and also to identify when differences in fecundity become apparent in the maturation cycle. There were variations in fecundity on both the temporal and spatial scales. The greatest variation in fecundity between years occurred in the western Irish Sea, whereas there was no variation between years in the southeastern Irish Sea (Liverpool Bay). There was no difference in fecundity between areas or years during September. The maximum fecundity in plaice is determined by the total weight of the fish at the end of follicle recruitment in the ovar...

Frequent skipped spawning in the world’s largest cod population

Life-history theory suggests that animals may skip reproductive events after initial maturation to maximize lifetime fitness. In iteroparous teleosts, verifying past spawning history is particularly difficult; the degree of skipped spawning at the population level therefore remains unknown. We unequivocally show frequent skipped spawning in Northeast Arctic cod (NEAC) in a massive field and laboratory effort from 2006 to 2008. This was verified by postovulatory follicles in temporarily arrested ovaries close to the putative spawning period. At the population level, “skippers” were estimated to be approximately equally abundant as spawning females in 2008, constituting ∼24% of the females 60–100 cm. These females never truly started vitellogenesis and principally remained on the feeding grounds when spawners migrated southward, avoiding any migration costs. The proximate cause of skipping seems to be insufficient energy to initiate oocyte development, indicating that skipped spawning may partly be a density-dependent response important in population regulation. Our data also indicate more skipping among smaller females and potential tradeoffs between current and future reproductive effort. We propose that skipped spawning is an integral life-history component for NEAC, likely varying annually, and it could therefore be an underlying factor causing some of the currently unexplained large NEAC recruitment variation. The same may hold for other teleosts.

Do protozoa contribute significantly to the diet of larval fish in the Irish Sea

This study evaluates the role of protozoa in larval fish feeding by describing protozoa in larval fish diets and testing the hypothesis that, in the Irish Sea, larval fish feed on protozoan prey at rates that potentially sustain their food requirements. Gut contents of 11 taxonomic groups of larval fish were examined, and protist prey occurred in the diet of all of them. Protozoan prey were identified, which provided an insight into their trophic role. Most of the protozoan prey were autotrophic or mixotrophic. In general, larval fish diets were constant over the spring/summer period, regardless of prey availability in the field and the composition of larval fish assemblage (taxonomy and size). A laboratory experiment on ingestion rates of flounder larvae as a function of ciliates concentration was conducted. Combined laboratory and field data showed that, in the Irish Sea, it is unlikely that ciliates are often the primary food source of flounder larvae, and, by implication, other larval fish as well. However, ciliates and other protozoa could be a substantial component of the larval fish diet, and they may potentially prevent food limitation.

Seasonal changes in a sandy beach fish assemblage at Porto Pim, Faial, Azores

Abstract A shallow-water fish assemblage, over a soft, sandy substratum at Porto Pim Bay on the island of Faial, Azores, was sampled at monthly intervals with a beach seine. Sampling was undertaken at 3-h intervals over 24 h on a monthly basis between July 1989 and June 1990. There was seasonal variation in the density of individuals and biomass with the highest values in summer (1·4 m −2 and 27·75 g m −2 respectively). A total of 24 species were observed and the assemblage was dominated by four species ( Chelon labrosus, Sardina pilchardus, Pagellus bogaraveo, Trachinotus ovatus ). Species diversity ( H ′) was high throughout the year but equatability ( J ) was low, due to the dominance of a few species. The largest change in the assemblage structure occurred between March and April. Only a few species could be classified as resident, with the two predators ( Dasyatis pastinaca and Echiichthys vipera ) being abundant at different times of the year. Most of the species observed were juvenile fish, many with an offshore distribution as adults, using Porto Pim Bay as a nursery ground.

Individual growth of juvenile plaice (Pleuronectes platessa L.) on a small irish sea nursery ground (Port Erin Bay, Isle of Man, UK)

Abstract The individual growth rate of juvenile plaice ( Pleuronectes platessa L. was studied in Port Erin Bay, Isle of Man, between 1989 and 1993. All plaice larger than 85 mm were measured and tagged with individually numbered anchor tags and released. All recaptured fish were remeasured and re-released back into the bay. The tagging data were used to estimate the population size between May and April each year and the growth rate of individual fish. The growth of juvenile plaice in Port Erin Bay was compared to theoretical growth rates which were modulated by temperature alone or a combination of temperature and initial length. These data were also compared to population level growth rates calculated from changes in mean length per month from regular beam trawl series. A total of 2547 plaice larger than 85 mm were tagged with 435 recaptures. Over the sampling period there were minor differences in temperature between years. The population growth rate varied between 0.1 and 0.27 mm·d −1 whereas the individual growth rate varied between 0 and 1.67 mm·d −1 . There were significant differences in growth rate both between seasons and between years. During the summer the individual growth rate was significantly higher than the models predicted. Low temperatures often coincided with high densities so that it was difficult to separate the effects of density and temperature on growth rate. At the individual level growth rate tended to be highest when the conditions on the nursery ground were optimal whereas at the population level there did not appear to be a simple relationship.

Regional variability in the dynamics of reproduction and growth of Irish Sea plaice, Pleuronectes platessa L

Abstract The differences in size-specific fecundity in relation to size/age at maturity, and in reproductive and somatic investment were analysed for female plaice caught in four regions of the Irish Sea (Cumbrian coast, Liverpool Bay, Cardigan Bay and western Irish Sea), each of which contains a spawning focus. Both the reproductive investment (gonad weight as a function of body size) and fecundity–size relationship of plaice in the western Irish Sea were significantly different from those in the other regions. Rates of annual somatic growth appeared to fall into three distinct groups (Cumbrian coast; Liverpool Bay and Cardigan Bay; western Irish Sea) and, in all cases, the rate of somatic growth fell rapidly after maturity. The data suggest that the highest surplus production (as spawned eggs) occurs in the sub-populations of plaice on the Cumbrian coast and in Liverpool Bay, and is linked to reduced intra-specific competition for food.