A. Moore

A critical life stage of the Atlantic salmon Salmo salar: behaviour and survival during the smolt and initial post‐smolt migration

The anadromous life cycle of Atlantic salmon Salmo salar involves long migrations to novel environments and challenging physiological transformations when moving between salt-free and salt-rich waters. In this article, (1) environmental factors affecting the migration behaviour and survival of smolts and post-smolts during the river, estuarine and early marine phases, (2) how behavioural patterns are linked to survival and (3) how anthropogenic factors affect migration and survival are synthesized and reviewed based on published literature. The timing of the smolt migration is important in determining marine survival. The timing varies among rivers, most likely as a consequence of local adaptations, to ensure sea entry during optimal periods. Smolts and post-smolts swim actively and fast during migration, but in areas with strong currents, their own movements may be overridden by current-induced transport. Progression rates during the early marine migration vary between 0.4 and 3.0 body lengths s(-1) relative to the ground. Reported mortality is 0.3-7.0% (median 2.3) km(-1) during downriver migration, 0.6-36% (median 6.0) km(-1) in estuaries and 0.3-3.4% (median 1.4) km(-1) in coastal areas. Estuaries and river mouths are the sites of the highest mortalities, with predation being a common cause. The mortality rates varied more among studies in estuaries than in rivers and marine areas, which probably reflects the huge variation among estuaries in their characteristics. Behaviour and survival during migration may also be affected by pollution, fish farming, sea lice Lepeophtheirus salmonis, hydropower development and other anthropogenic activities that may be directly lethal, delay migration or have indirect effects by inhibiting migration. Total mortality reported during early marine migration (up to 5-230 km from the river mouths) in the studies available to date varies between 8 and 71%. Hence, the early marine migration is a life stage with high mortalities, due to both natural and human influences. Factors affecting mortality during the smolt and post-smolt stages contribute to determine the abundance of spawner returns. With many S. salar populations in decline, increased mortality at these stages may considerably contribute to limit S. salar production, and the consequences of human-induced mortality at this stage may be severe. Development of management actions to increase survival and fitness at the smolt and post-smolt stages is crucial to re-establish or conserve wild populations.

Locations of marine animals revealed by carbon isotopes

Knowing the distribution of marine animals is central to understanding climatic and other environmental influences on population ecology. This information has proven difficult to gain through capture-based methods biased by capture location. Here we show that marine location can be inferred from animal tissues. As the carbon isotope composition of animal tissues varies with sea surface temperature, marine location can be identified by matching time series of carbon isotopes measured in tissues to sea surface temperature records. Applying this technique to populations of Atlantic salmon (Salmo salar L.) produces isotopically-derived maps of oceanic feeding grounds, consistent with the current understanding of salmon migrations, that additionally reveal geographic segregation in feeding grounds between individual philopatric populations and age-classes. Carbon isotope ratios can be used to identify the location of open ocean feeding grounds for any pelagic animals for which tissue archives and matching records of sea surface temperature are available.

Use of the Stable Isotope Composition of Fish Scales for Monitoring Aquatic Ecosystems

Publisher Summary The structure and function of ocean ecosystems are clearly linked to climate, and it is likely that changes in climate due to increased warming or perturbations of natural climate forcing cycles will similarly impact marine ecosystem function. All organisms within a marine food web ultimately derive carbon and nitrogen from primary production, and the isotopic composition of a consumers tissues will be averaged over the time taken to grow the sampled tissue. Fish represent attractive isotopic integrators as commercial fish are routinely sampled in huge numbers from all of the worlds oceans. It is important to note that fish are mobile organisms, and the interpretation of fish isotope signals is limited by the knowledge of the area sampled. Scales provide a particularly attractive target tissue for isotopic analysis as they are easily sampled and stored. Most importantly, the scales of commercially exploited fish are routinely sampled to establish age and population structure, and in many cases these scales are kept in archived collections, often covering more than 60 years. The retrospective analysis of the isotopic composition of archived fish scales can therefore provide information concerning temporal changes in ecosystem structure and function. Scales also present an attractive option for the long-term monitoring of aquatic ecosystems in the future. The purpose of this chapter is to briefly review the potential for using the stable isotope composition of archived fish scales, to provide decadal records of variability in ecosystem structure, in both freshwater and marine ecosystems.

Magnetic particles associated with the lateral line of the European eel Anguilla anguilla

Magnetization measurements of the European eel Anguilla anguilla demonstrated the presence of magnetic material concentrated in the region of the mandibular canals of the lateral line system. The data suggest that the material is magnetite, has a size suitable for magnetoreception and is of biogenic origin. The presence of magnetic particles in the lateral line system is discussed in relation to their possible role in allowing the fish to orientate with respect to the geomagnetic field during their extensive oceanic spawning migrations.

The effects of a small hydropower scheme on the migratory behaviour of Atlantic salmon Salmo salar smolts

The potential effects of a hydropower scheme on the migratory behaviour of Atlantic salmon Salmo salar smolts was studied on the River Frome, southern England. The potential delay to migration at the intake of the hydropower scheme was assessed, together with the effects of passage through the turbine on the temporal and spatial migration of the fish in the river and estuary. The migratory behaviour of the emigrating S. salar smolts was monitored using miniature acoustic transmitters and an array of acoustic receivers positioned at the hydropower scheme and in the river and estuary. The majority of the smolts bypassed the hydropower scheme with only 8.1% of the fish moving downstream through the turbine. Movement was nocturnal and occurred during elevated river flows. There was no apparent delay at the turbine intake or at the adjacent weir. The subsequent migration of all smolts through the estuary of the River Frome occurred during both day and night and there was a distinct ebb-tide migration through the estuary and into the coastal zone. There was no difference in the rate of migration between smolts that moved through the turbine or over the weir. The detection of smolts during both the freshwater migration and the transition from the freshwater to the marine environments was high (91.8 and 73.3%, respectively). A laboratory investigation on the de-scaling of smolts indicated that removal of 1, 5 and 10% of scales had no significant effect on saltwater survival or the measured physiological parameters (gill Na+ -K+ -ATPase activity, plasma osmolality and chloride concentrations). Smolt passage through the turbine was assessed and resulted in either no damage to the integument or scale loss or between 20 and 80% of total body area of recaptured smolts. It is estimated that 1.53% of the smolt population would suffer significant damage after passage through the turbine. The implications of the hydropower scheme on the population of salmon in the River Frome are discussed.