Estimates Of Year-class Strength Research Articles

Participatory place-based integrated ecosystem assessment in Sitka, Alaska: Constructing and operationalizing a socio-ecological conceptual model for sablefish (Anoplopoma fimbria)

An integrated ecosystem assessment (IEA) is a framework that organizes science to aid in the transition from traditional single-species management towards ecosystem-based management. Within the Gulf of Alaska, we started a small-scale IEA framework with active engagement of local stakeholders in the fishing community of Sitka. The initial step of this framework was the development of conceptual models. Here, we present a co-produced conceptual model using data gathered from participatory focus groups and a literature review of ecosystem attributes driving the abundance of sablefish (Anoplopoma fimbria). We then assembled a qualitative network model (QNM), a mathematical representation of a conceptual system in which perturbations of individual components can be assessed for their qualitative effect. Simulations were conducted to test different biophysical scenarios, while evaluating tradeoffs across ecological and human dimension components. In contrast to large adult sablefish, juvenile and small adults responded similarly to most biophysical conditions. In general, human dimension components responded negatively to the tested scenarios. These results highlight the potential need for management strategies that differentiate between small and large adults, particularly if current sablefish stock assessment trends persist, such as the lack of older fish contributing to spawning biomass and uncertainties in estimates of year-class strength.

Can migrants escape from density dependence?

AbstractMigration is thought to maximize growth by enabling individuals to escape from density dependence, but this has rarely been tested at the individual level in natural populations. We employed linear mixed modeling of the spacing between consecutive scale growth rings to reconstruct individual growth profiles of a paradigmatic fish migrant, the sea trout (Salmo trutta) and related these to estimates of year class strength over a 13‐year period. Variation in scale growth was 1.3 times greater among individuals than within individuals in freshwater and 10 times greater at sea. Scale growth was inversely related to year class strength, both in freshwater (before migration) and at sea (after migration). Competition for patchily distributed resources is the most plausible explanation of the negative density‐dependent growth observed in freshwater and, to a lesser extent, in the marine environment. Our study provides some of the strongest evidence for a role of density dependence in determining partial migrations because although migrants can maximize growth by moving into the sea, they do not appear to become free from density dependence constraints completely. This has implications for conservation and suggests that sea trout and other anadromous fish displaying partial migrations may not be best managed on a river by river basis, but rather from a broader, coastal perspective.

Winter winds and river discharge determine juvenile southern flounder ( Paralichthys lethostigma) recruitment and distribution in North Carolina estuaries

Retrospective analyses of a 23 year data set on abundance of Age 0 southern flounder in 105 estuarine nursery areas in the coastal region of North Carolina showed that discernible temporal and spatial patterns exist among clusters of stations. Furthermore, these patterns could be quantitatively related to certain meteorological and hydrological variables, namely winds from the east–southeast (E–SE) and from the north–northeast (N–NE) sectors and river runoff, which explained up to 83% of the interannual variability in numbers. We developed a regression model using recent catch data (1987–2002) and used the model to hindcast an earlier segment of the time series (1979–1986). The model was found to be quite robust, and could predict year class strength within 1 to 80% in the test set of data. We interpret these results to mean that hydrodynamic factors are principally responsible for the observed interannual recruitment variability in southern flounder in NC, since the interannual pattern in abundance of Age 0 fish persists for 2 more years of adult life. Finally, we discuss the implications of the variable spatial distribution patterns for estimates of year class strength from juvenile abundance data. It is possible that estimates of year class strength with a useful level of confidence could be obtained from meteorological data during the larval migration period.

Recruitment variability in dab ( Limanda limanda) in the southeastern North Sea

Data from three annual surveys, covering inshore and offshore waters of the southeastern North Sea, were analysed to study recruitment variability in dab ( Limanda limanda) over the period 1978–1997. Geometric mean abundance of 0- to 5-group dab was estimated using general linear models. Juvenile dab (0- and 1-group) were found over the entire area, from inside the estuaries to 50 m depth offshore. Environmental conditions (water temperature, wind stress, turbidity) affected the catch rates. The potential errors in the estimates of year-class strength, caused by differences in catchability, are discussed. The inter-annual pattern of year-class strength appeared to be established between ages 1 and 2, suggesting that factors determining recruitment are not restricted to the pelagic early life phase only, but also operate during the demersal juvenile phase. Recruitment variability at age 2 was in the order of 50–60% and appears to be equal to, or lower than, recruitment variability in plaice and sole. These results contradict expectations based on the concentration hypothesis, which states that the degree of variation in recruitment is inversely related to the degree of concentration during early life phases.

Determining abundance, distribution, and mortality of 0-group plaice (Pleuronectes platessa L.) in the Wadden Sea

Summary Errors in estimates of year class strength and mortality among 0-group plaice can result from migrations. In order to reliably determine the number present, tidal flats and channels in a large and clearly delimited Wadden Sea area (715 km2) were sampled simultaneously using different fishing methods (by-catch, beam trawls, pushnets, and fyke nets). The numbers of 0-group plaice calculated for each depth stratum reflected changes in the distribution patterns during the complete diurnal cycle and during the course of the season and proved to be most reliable for estimating the total number in the whole area. In contrast to this, the traditional way of calculating population size by multiplying the arithmetic mean of simultaneous catches resulted in very different numbers which would entail an overestimation of mortality.

Environmental Influences on Early Development and Year-Class Strength of Northern Pike in Lakes Oahe and Sharpe, South Dakota

Abstract Survival of artificially-fertilized ova and larvae of northern pike, Esox lucius, was estimated from embryos held in natural spawning areas in Lake Oahe and Lake Sharpe, two main stem Missouri River reservoirs. Mortalities approaching 100% during early embryonic development were associated with sudden drops in water temperature below 10 C or prolonged temperatures near 5 C. Silt deposition of 1.0 mm per day was associated with mortality of 97% or above. Following hatching available food appeared to be a more important factor in survival than temperature change or silt deposition. Estimates of year-class strength of northern pike in the two reservoirs suggest that large year classes were associated with stable to rising water level and temperature, flooded vegetation, and calm weather during the spawning season. Small year classes have been associated with abrupt water temperature fluctuations, dropping water level, and high silt deposits.