Jens-Peter Herrmann

Short-term decoupling of otolith and somatic growth induced by food level changes in postlarval Baltic sprat, Sprattus sprattus

We studied the effects of food level changes on otolith and somatic growth in postlarval Baltic sprat reared initially for a period of 11 days under zero, low, and ad libitum feeding conditions. During a subsequent 11 day period, feeding regimes were reversed in half of the low and ad libitum feeding treatments, and starved fish were re-fed ad libitum rations. Somatic growth rates under low and ad libitum food rations ranged between 0.15-0.22 mm day -1 and 0.48-0.63 mm day -1 , respectively, and led to significant differences in length and weight between feeding regimes. Previously starved fish, however, grew only 0.25-0.28 mm day -1 under ad libitum conditions. During the first period, significant linear relationships were found for otolith v. length and v. weight growth across all treatments. After changing feeding regimes, increment widths failed to significantly predict somatic growth for 9 days, after which a significant relationship between otolith and somatic growth became re-established. Recent otolith growth was a good predictor of fish condition after the first, but not after the second period. The results suggest that perturbations in environmental conditions can temporarily decouple otolith from somatic growth in postlarval sprat, which needs to be considered in field studies.

Estimating post-release mortality of European sea bass based on experimental angling

Estimating post-release mortality of European sea bass based on experimental angling Wolf-Christian Lewin, Harry Vincent Strehlow, Keno Ferter, Kieran Hyder, Jan Niemax, Jens-Peter Herrmann, and Marc Simon Weltersbach* Thünen Institute of Baltic Sea Fisheries (Thünen-OF), Alter Hafen Süd 2, 18069 Rostock, Germany Institute of Marine Research, Fisheries Dynamics, PO Box 1870 Nordnes, 5817 Bergen, Norway Centre for Environment, Fisheries and Aquaculture Science, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK Centre for Earth System Research and Sustainability (CEN), Institute for Hydrobiology and Fisheries Science, University of Hamburg, Klimacampus, Olbersweg 24, 22767 Hamburg, Germany Institute for Biosciences, University of Rostock, Albert-Einstein-Straße 3, 18059 Rostock, Germany *Corresponding author: tel: þ49 381 8116 128; fax: þ49 381 8116 199; e-mail: simon.weltersbach@thuenen.de

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.

Effect of fish length and nutritional condition on the fecundity of distressed Atlantic cod Gadus morhua from the Baltic Sea: Potential fecundity of Baltic G. Morhua

The disappearance of larger individuals and the decrease in individual body condition suffered by Atlantic cod Gadus morhua in the eastern Baltic during the past two decades can be expected to affect the stock reproductive output. To investigate this, female G. morhua were collected during the spawning and pre-spawning period in 2015-2016. The current individual potential fecundity (FP ) of eastern Baltic G. morhua was estimated and analysed in relation to total length (LT ) and indices of nutritional status such as body condition (K) and hepato-somatic index (IH ) using generalized linear models. In addition, the current prevalence of atresia and its potential relation to K were investigated. Moreover, a calibration curve to estimate FP from oocyte diameter, based on the autodiametric oocyte counting method, was established for the first time for eastern Baltic G. morhua and can be used for future fecundity studies on this stock. The results showed that FP was mainly positively related to fish length, but K and IH also contributed significantly to the variation in FP . The model predicted that fish with K = 1·2 have a FP 51% higher than fish of the same LT with K = 0·8. The prevalence of fecundity regulation by atresia was 5·8%, but it was found only in fish in the pre-spawning maturity stage and with low K. Temporal changes in biological features such as the length composition and individual body condition of eastern Baltic G. morhua, should be accounted for when estimating stock reproductive potential.

Ignoring the vertical dimension: biased view on feeding dynamics of vertically migrating sprat (Sprattus sprattus)

Original Article Ignoring the vertical dimension: biased view on feeding dynamics of vertically migrating sprat (Sprattus sprattus) Rini Kulke, Viola Bödewadt, Kristin Hänselmann, Jens-Peter Herrmann, and Axel Temming* Institute for Hydrobiology and Fisheries Science, University Hamburg, Olbersweg 24, 22767 Hamburg, Germany *Corresponding author: tel: þ49 40 42838 6617/6620; fax: þ49 40 42838-6618; e-mail: atemming@uni-hamburg.de