Do habitat measurements in the vicinity of Atlantic salmon (Salmo salar) parr matter?

Atlantic salmon, Salmo salar L., parr habitat characterisation is usually performed by in situ measures of key environmental variables taken at the exact fish location if the fishing gear allows precise pinpointing of this location, or in large sampling sections covering a river reach or mesohabitat, often ignoring variability in the immediate vicinity around individual fish. These data may be critically important in the development and validation of habitat preference models. The influences of seven increasing distances of measures, the variation of the number of considered measures and the depth of velocity measurement (bottom or 0.6 of the depth) in the calculations of HSI (Habitat Suitability Index) from a multiple‐experts fuzzy model of Atlantic salmon parr habitat were tested. When a parr was present, six measures collected in a 50‐cm radius around the fish to provide an average measure as input data and velocity measured at 60% of the depth gave the highest HSI values. These results show some potential for the use of an intermediate study scale, between micro‐ and mesohabitat, and questions how fish habitat conditions are currently measured.     

Abundance estimation of Northeast Atlantic mackerel based on tag recapture data—A useful tool for stock assessment?

In the present study we utilize tag recapture data to estimate year class abundance and spawning stock biomass of mackerel (Scomber scombrus L.) in the Northeast Atlantic for the period 1986–2008. On average 20,000 jigged mackerel have been tagged annually with internal steel tags in the spawning area west of Ireland and the British Isles, and the tags have been recaptured in commercial catches screened through metal detectors. The spawning stock biomass estimates derived from two different tag-based models were highly variable but were on average 2 and 2.3 times higher than the ICES official estimate. The official estimate is considered uncertain and most likely an underestimate of the actual biomass, due to unregistered mortality in the fisheries and lack of fishery-independent, age-disaggregated data. Hence, tag-based estimates could potentially improve the current assessment if included in the ICES stock assessment on a regular basis. These estimates also involve some uncertainty that needs consideration, especially related to variable tagging mortality, detector efficiency and migrations of the stock.     

Do albacore exert top‐down pressure on northern anchovy? Estimating anchovy mortality as a result of predation by juvenile north pacific albacore in the California current system

Quantifying the mortality of marine fishes is important for understanding spawner–recruit relationships, predicting year‐class strength, and improving fishery stock assessment models. There is increasing evidence that pelagic predators can exert a top‐down influence on prey, especially during critical early life‐history stages. The objective of this study was to quantify predation by North Pacific albacore on Northern anchovy in the California current system (CCS). I estimated the abundance of juvenile albacore in the CCS from 1966–2005 using stock assessment models and spatially explicit catch‐per‐unit‐effort time series. Anchovy abundance (1966–93), both recruitment and total biomass, was obtained from a stock assessment model. Annual rates of anchovy consumption by albacore were calculated using diet studies of albacore in the CCS, an age‐structured bioenergetics model, and regional estimates of albacore abundance. The range of estimates was large: albacore may remove from less than 1% to over 17% of anchovy pre‐recruitment biomass annually. Relationships between predation and recruitment biomass were consistent with expectations from top‐down effects, but further study is required. This is the first attempt to quantify a specific source of mortality on anchovy recruits and to demonstrate potential top‐down effects of predation on anchovy.     

Decline and fall of the salmon fisheries in the Netherlands: is restocking the Rhine a reality?

Abstract. The Atlantic salmon, Salmo salar L., was once of major importance to the fishing industry along the River Rhine. This paper discusses the Dutch and German salmon catches over the years 1863–1950. Even up to the end of the last century, it was not uncommon for Dutch and German fishermen to land 100000 salmon a year. However, factors such as the increased use of locks and weirs along the Rhine, coupled with growth in pollution, soon led to a rapid decline in numbers. By 1933, the salmon fishing industry in the Netherlands had virtually ceased to exist. Analysis of the available catch statistics suggests that the decline in the salmon population could have started before official records began. Although the degree of scatter in the data and uncertainties in the assumptions preclude the possibility of drawing firm conclusions about the survival rate of salmon, these figures illustrate how difficult it will be to maintain a stable population in the Rhine. Moreover, a number of changes have taken place since the heyday of salmon in western Europe, which could compound the problem. Of particular importance in the context of the Rhine are:

• 1 the closure of two of the major migration routes to the sea (Haringvliet and Zuiderzee);
• 2 morphological changes in the river;
• 3 chemical and thermal pollution;
• 4 the loss of accessible spawning and nursery areas of the required quality;
• 5 the disappearance of salmon from other rivers that flow into the North Sea such as the Rivers Elbe, Weser and Ems. If salmon were only reintroduced into the Rhine, a certain proportion would probably stray and infiltrate these other rivers.

The fact that the impact of these changes is difficult to quantify increases the uncertainty associated with maintaining a stable stock of salmon in the Rhine.     

Why is the antibody response of Atlantic cod so poor? The search for a genetic explanation

ABSTRACT:   The immune system of the Atlantic cod ( Gadus morhua L) is unusual in that it cannot produce a specific antibody response upon immunization. Despite this, the cod is not particularly susceptible to infectious disease in its normal environment. This review examines the potential genetic basis for the lack of a specific antibody response in the cod. The genetics of cod immunoglobulins are compared with those of other well-characterized teleost fish. A review of the evidence suggests that deficiencies in the number, structure, organization, diversity and expression of both Immunoglobulin (Ig) heavy and Ig light chain genes in cod cannot explain its unusual antibody response. It is concluded that a deficiency in cod major histocompatibility (MH) class II molecules is a prime suspect for the lack of a specific antibody response in cod, and that testing of this hypothesis must await future experimentation.     

Does the capacity for energy utilization affect the survival of post‐smolt Atlantic salmon,Salmo salar L., during natural outbreaks of infectious pancreatic necrosis?

If osmotic stress and reduced seawater tolerance are predisposing factors for infectious pancreatic necrosis (IPN) outbreaks in farmed Atlantic salmon, increased survival by enhancing access to energy would be expected. The aim of the present study was, therefore, to increase energy access in 1-year old Atlantic salmon after sea transfer by increasing the level of dietary fat, by exchanging some of the dietary oil with more easily oxidized medium chain triacylglycerols, or by dietary supplementation of potentially energy enhancing additives such as clofibrate and tetradecylthioacetic acid (TTA). A natural outbreak of IPN occurred 8 weeks after sea transfer, and a significant dietary effect explaining 76% of the variation in mortality was observed. Relative percentage survival for the fish fed TTA in sea water was 70% when compared with the unsupplemented control, reducing mortality from 7.8 to 2.3%. Muscle fat content and plasma chloride were related to IPN mortality, suggesting that reduced hypoosmoregulatory capacity might be a predisposing factor to the onset of an IPN outbreak. Based on the observation of a threefold increase in white muscle mitochondrial fatty acid oxidizing activity by TTA, it is suggested that TTA has resulted in a re-allocation of dietary fatty acids from storage to energy producing oxidation.     

Spatial and temporal distribution of Lepeophtheirus salmonis (Kr?yer) larvae in a sea loch containing Atlantic salmon, Salmo salar L., farms on the north-west coast of Scotland.

A plankton survey investigated spatial and temporal distributions of larval sea lice in a sea loch on the north-west coast of Scotland. Plankton tows were collected approximately weekly at two depths (0 and 5 m) at five sample stations over a 2-year period. The survey began in March 2002 when Atlantic salmon, Salmo salar L., farms in the study area stocked with fish and finished in March 2004, after farms completed harvesting. Over 90% of lice recovered were Lepeophtheirus salmonis (Kr?yer); therefore, the results of this paper refer largely to that species. The data were analysed using generalized additive models with explanatory variables for time, depth and station. Greatest densities of nauplii were recovered at stations adjacent to farms indicating that the local salmon farms were a likely source of larvae at times during the production cycle. There were significant temporal trends in larval densities and the stocking and harvesting of farms can possibly account for these increases and decreases, particularly at the start and at the end of the production cycle. The distribution of copepodids was more widespread than that of nauplii, indicating that the larvae can be transported several kilometres from the point of release.     

Is the decline of freshwater mussel populations in a regulated coastal river in south‐eastern Australia linked with human modification of habitat?

• 1. The Hawkesbury‐Nepean River provides potable water for 5 million people living in the Sydney basin, and water for agricultural and horticultural production that meets most of Sydney's daily needs for fresh food. Anecdotal evidence indicated that numbers of freshwater mussels have seriously declined in much of the river over recent decades.
• 2. A field survey revealed the presence of populations of three species of mussels, Hyridella depressa, Hyridella australis and Velesunio ambiguus in the river. Higher density mussel populations were most common in catchment areas with little human modification to the channel bed or adjacent vegetation communities.
• 3. Levels of disturbance of riparian vegetation and, to a lesser degree, land use, were identified as being strongly associated with the absence of mussels from some reaches.
• 4. Catchment geomorphology was also shown to be relevant to the abundance, population structure and suite of mussel species present in different geomorphic reaches of the river. Absence of mussels was noted from areas where they had been recorded in previous studies. These results demonstrate that mussel species are under threat in the catchment.
• 5. As filter feeders with the ability to remove excess nutrients and bioaccumulate toxic substances, freshwater mussels play an important role in natural remediation processes in freshwater systems.
• 6. The decline in mussel populations in the Hawkesbury‐Nepean River highlights concerns for the overall health of the river system, and supports the need to identify the subsidiary impacts of physical habitat modification in developing both riverine and riparian management strategies.

Copyright © 2006 John Wiley & Sons, Ltd.     

Direct contact between Pseudo-nitzschia?multiseries and bacteria is necessary for the diatom to produce a high level of domoic acid

Pseudo-nitzschia multiseries loses most of its ability to produce domoic acid when it is cultured axenically. However, domoic acid production recovers when bacteria from the original culture are reintroduced to axenic cultures, indicating a bacterial association with domoic acid production in this species. In order to determine the role of bacteria in domoic acid production, an axenic strain prepared from a nonaxenic strain of P. multiseries was inoculated into media in cellophane tubes, which were then placed in a bottle containing the original nonaxenic culture. Both strains showed characteristic domoic acid production similar to that previously observed. Domoic acid level in the cells of the axenic strain in the tubes was much smaller than that of the cells outside the tubes. These results show that direct contact between living bacteria and P. multiseries is necessary for producing high levels of domoic acid in P. multiseries.