Inigo Everson

Spatial and temporal variation in the distribution and density of pelagic fish species in Lake Victoria, East Africa

The distribution and densities of three pelagic fish taxa (Nile Perch, Lates niloticus, Rastrineobola argentea [Dagaa], and the haplochromine cichlids) in Lake Victoria were estimated through 17 lake-wide acoustic surveys conducted in two series (August 1999–August 2002 and August 2005–September 2011). Nile Perch densities were estimated through echo-counting, while Dagaa and haplochromines by echo-integration. Mixed generalized linear model estimates indicated up to 30% decline in Nile Perch densities in the deep and coastal areas and up to 65% reduction in the shallow inshore areas over the study period. There was a twofold increase in Dagaa densities and a 10% increase in haplochromines. The distribution and densities of these species were influenced by season, stratum and year of survey. In addition to fish exhibiting seasonal clustering in the upper layers of the water column, they also appeared to spread to shallow inshore waters. The Nyanza, Speke, and Emin Pasha Gulfs demonstrated diverse spatial stock oscillations in abundance, and distribution which call for a need to include area disaggregated considerations in stochastic models when predicting dynamics of fish stocks.

Diet and growth rates of Meganyctiphanes norvegica in autumn

Abstract Fatty acid biomarkers analysis was performed on juvenile and adult northern krill (Meganyctiphanes norvegica) to determine variability in diet between individuals and the consequences for their growth rates. Animals were sampled from Gullmarsfjorden, western Sweden during September 2004 and incubated for 4 days to measure instantaneous growth rate (IGR) before subsequent fatty acid analysis. We found positive linear relationships between IGR and certain fatty acid concentrations within adult specimens. In particular, highest growth was seen in individuals containing high concentrations of 18:2(n-6) and 18:3(n-6), indicative of filamentous algae and/or terrestrial carbon dietary sources, and 16:4(n-1), a biomarker fatty acid for diatoms. Similar relationships were not evident for juvenile krill. In an accompanying study, stomach content analyses established that adult krill had been feeding on a diverse range of red, brown and green benthic filamentous algae, which contain fatty acids of the (n-6) series. Incidence of filamentous algae in the stomachs of juvenile krill was much lower, possibly reflecting the shallower water depths inhabited by this maturity stage. Although a benthic feeding mode has previously been observed for M. norvegica, this is the first direct evidence to indicate that benthic food sources can be important in supporting the growth of this euphausiid. Furthermore, high variability in growth rates and fatty acid concentrations between individuals suggest a diversity of feeding strategies within this fjordic population.

Spatial patterns in the vertical structure of euphausiids in Gullmarsfjord, Sweden: Identifying influences on bilayer formation and distribution

Abstract The formation of two vertically discrete layers (bilayers) at night-time is a commonly observed phenomenon in zooplankton and is regularly found in Gullmarsfjord, a fjord with a 50 m sill depth, deep basin and a three-layered water column. In an acoustic and net sampling survey in September 2003, night-time euphausiid layers occurred at 15 and 45 m, with the deeper layer containing relatively higher concentrations of adult Northern krill (Meganyctiphanes norvegica). The main night-time predatory threat came from the upward migration of demersal fish, which reached the deeper but not the shallower euphausiid layer. Shoreward advection of coastal waters across the sill creates a layer of resuspended organic matter between 40 and 50 m. The deeper bilayer was located at those depths, particularly at the mouth of the fjord where this organic matter was most concentrated. Krill in the lower bilayer experienced waters that were 4°C cooler than in the upper bilayer, which can decrease the cost of respiration by around 20%. Accompanying studies have shown significantly higher growth rates in krill consuming sedimentary organic material and benthic filamentous algae. Combined, it appears that energetic benefit and predatory threat were greatest in the deeper rather than the shallower bilayer in Gullmarsfjord. This is the reverse of most other euphausiid habitats, where the highest risk and reward is in the upper bilayer, illustrating that euphausiids adapt their stereotypic vertical migration pattern to local environmental conditions.