Marc Hufnagl

Hydrostatic pressure affects selective tidal stream transport in the North Sea brown shrimp (Crangon crangon)

ABSTRACT The brown shrimp (Crangon crangon) is a highly abundant invertebrate in the North Sea, with its life cycle stages ranging from deep offshore spawning to shallow onshore nursery areas. To overcome the long distances between these two habitats, brown shrimp are suspected to use selective tidal stream transport (STST), moving with the cyclic tide currents towards their preferred water depths. However, it is not known which stimulus actually triggers STST behavior in brown shrimp. In this work, we determined the influence of different hyperbaric pressures on STST behavior of juvenile brown shrimp. Brown shrimp activity was recorded in a hyperbaric pressure chamber that supplied constant and dynamic pressure conditions simulating different depths, with and without a tidal cycle. Subsequent wavelet and Fourier analysis were performed to determine the periodicity in the activity data. The results of the experiments show that STST behavior in brown shrimp varies with pressure and therefore with depth. We further show that STST behavior can be initiated by cyclic pressure changes. However, an interaction with one or more other environmental triggers remains possible. Furthermore, a security ebb-tide activity was identified that may serve to avoid potential stranding in shallow waters and is ‘remembered’ by shrimp for about 1.5 days without contact with tidal triggers. Summary: North Sea brown shrimp perform selective tidal stream transport (STST) activity under the influence of cyclic pressure and ‘remember’ the tidal cycle from the field for 1.5 days.

Exploring the microzooplankton–ichthyoplankton link: a combined field and modeling study of Atlantic herring (Clupea harengus) in the Irish Sea

The microzooplankton–ichthyoplankton link remains poorly resolved in field studies due to a lack of simultaneous sampling of these predators and potential prey. This study compared the abundance, distribution and growth of larval Atlantic herring (Clupea harengus) and the abundance, biomass and composition of micro- and small mesozooplankton throughout the Irish Sea in November 2012 and 2013. In contrast to warmer months, microzooplankton biomass was highest in eastern areas, in the vicinity of the main spawning grounds of herring. Although the protozoan composition differed somewhat between years, dinoflagellates (e.g. Gymnodinium spp., Protoperidinium spp., Ceratium furca) dominated in abundance and/or biomass, similar to other temperate shelf seas in autumn/winter. Spatial differences in the protozoan community were strongly related to hydrographic characteristics (temperature, salinity). Significant relationships between the abundance of larval herring and dinoflagellates (positive) and copepodites (negative) suggested that complex grazing dynamics existed among lower trophic levels. When different, in situ size fractions of zooplankton were used as prey in a larval herring individual-based model, simulations that omitted protozooplankton under-predicted observed (biochemically-based) growth of 8–18 mm larvae. This study suggests that small planktonic organisms (20–300 µm) should be routinely surveyed to better understand factors affecting larval fish feeding, growth and survival.

Predictive framework for codend size selection of brown shrimp (Crangon crangon) in the North Sea beam-trawl fishery

The brown shrimp (Crangon crangon) fishery is of great socio-economic importance to coastal communities on the North Sea. The fishery is exploited by beam trawlers often using codends with very small mesh sizes, leading to concerns about catch rates of undersized shrimp. However, little information is available on codend size selection, making it difficult to provide scientifically based advice on alternative codend designs. Therefore, this study establishes a predictive framework for codend size selection of brown shrimp, based on a large selectivity dataset from 33 different codend designs tested during four experimental fishing cruises, during which more than 350,000 brown shrimp were length measured. Predictions by the framework confirm concerns about the exploitation pattern in the fishery, because the retention probability of undersized shrimp reaches 95% with the currently applied designs. The framework predictions allow the exploration of obtainable exploitation patterns depending on codend design. For example, increasing codend mesh size to 25–29 mm would reduce the retention rate of undersized shrimp to a maximum of 50%, depending on codend mesh type.

Not easy to catch: multiple covariates influence catch rates of brown shrimp (Crangon crangon L.), potentially affecting inferences drawn from catch and landings data

Katharina Friederike Schulte*, Andreas Dänhardt, Marc Hufnagl, Volker Siegel, Werner Wosniok, and Axel Temming Thünen Institute (Federal Research Institute for Rural Areas, Forestry and Fisheries), Institute of Sea Fisheries, Palmaille 9, 22767 Hamburg, Germany Institute for Hydrobiology and Fisheries Science, University of Hamburg, Olbersweg 24, 22767 Hamburg, Germany Institute of Statistics, University of Bremen, Linzer Straße 4, 28359 Bremen, Germany *Corresponding author: tel: þ49 421 67301819; fax: þ49 403 8905263; e-mail: katharina_schulte@gmx.de.