Have climate induced changes in zooplankton communities led to poor conditions for larval fish growth? A test on cod larvae on the UK shelf

Climate effects have been shown to be at least partly responsible for the reorganisation in the plankton ecosystem on the shelf seas of NW Europe over the last 50 years. Most fish larvae feed primarily on zooplankton, so changes in zooplankton quantity, quality and seasonal timing have been hypothesized to be a key factor affecting their survival. To investigate this we have implemented a 1-dimensional trophodynamic growth model of cod larvae for the waters around the UK covering the period 1960 to 2003. Larval growth is modelled as the difference between the amount of food absorbed by the larva and its various metabolic costs. Prey availability is based upon the biomass and size of available preys (i.e. adults and nauplii copepods and cladocerans) taken from the Continuous Plankton Recorder dataset. Temperature and wind forcing are also taken into account. Results suggest that observed changes in plankton community structure may have had less impact than previously suggested. This is because changes in prey availability may be compensated for by increased temperatures resulting in little overall impact on potential larval growth. Stock recovery, at least in the short term is likely to be more dependent upon conserving the year classes recruited to allow spawning stock biomass to rebuild. If as our model suggests, the larvae are still able to survive in the changing environment, reduction in fishing on the adults is needed to allow the stock to recover.

Acceptance angle effects on the beam attenuation in the ocean

The beam attenuation serves as a proxy for particulate matter and is a key parameter in visibility algorithms for the aquatic environment. It is well known, however, that the beam attenuation is a function of the acceptance angle of the transmissometer used to measure it. Here we compare eight different transmissometers with four different acceptance angles using four different deployment strategies and sites, and find that their mean attenuation values differ markedly and in a consistent way with instrument acceptance angle: smaller acceptance angles provide higher beam attenuation values. This difference is due to variations in scattered light collected with different acceptance angles and is neither constant nor easy to parameterize. Variability (in space or time) in the ratios of beam attenuations measured by two different instruments correlates, in most cases, with the particle size parameter (as expected from Mie theory), but this correlation is often weak and can be the opposite of expectations based on particle size changes. We recommended careful consideration of acceptance angle in applications of beam transmission data especially when comparing data from different instruments. (C) 2009 Optical Society of America