Predator impacts on inshore Baltic herring (Clupea harengus) larvae: Lions, tigers and bears – but where? P. Kotterba1*, D. Moll1, L. von Nordheim1, C. Hammer1, M. A. Peck2, D. Oesterwind1, and P. Polte1 1Thünen-Institute of Baltic Sea Fisheries, Rostock, Germany; 2University of Hamburg, Germany

Background Predation on marine fish larvae is considered to be a major cause of their mortality. For example, in offshore retention areas of Pacific and Atlantic herring (Clupea spp.) larvae, a broad variety of different predators such as pelagic fish species and gelatinous plankton (Scyphozoa and Ctenophora) is assumed to significantly affect the survival of herring larvae. Furthermore, herring itself has been documented to conduct filial cannibalism. Alike other herring subpopulations, Baltic herring usually spawns in transitional waters and lagoons that are characterized by a different set of environmental factors and potential larvae predators. Are top-down mechanisms observed in offshore areas then transferable to inshore systems at all? Study site Greifswald Bay (54.2°N; 13.5°E) is part of the German Baltic coast and com- prises an area of 514 km². Average Filial cannibalism depth: 5.8 m; Salinity: 7 - 9; tidal ampli- by adult tudes < 10 cm. The bay is an important herring? demersal spawning area for Baltic herring. eggs adults

hatchlings

2 - Greifswald Baltic years 3

Bay Sea

growing Predation by the larvae resident fish community?

juveniles

Predation by Methods gelatinous plankton? Different sampling methods were applied to investigate the abundance of herring larvae and their predators (spatio-temporal overlap, see figure 1). Simultaneously, we collected stomachs of potential predators to determine the share of larvae therein: Herring larvae: Oblique Bongo net hauls at 36 distinct stations (25 years data series) & trawls with a modified epibenthic sledge in the littoral zone (2011) Potential predators: Bongo net bycatches; frame trawl catches in the bay (2011); beach seine hauls in the littoral (2011-2015). Adult herring were sampled with a gill net (2010-2015). Stomach samples were immediately fixated onboard (frozen on dry ice or with 4% formalin) & later analyzed in the laboratory.

Results adult herring fish community gelatin. plankton predator group:

spatio-temporal strong overlap temporal strong overlap overlap? (see figure 2) mismatch nearly no larvae nearly no larvae larvae predation? no predation in the stomachs in the stomachs

100 10 Conclusions

)

3

-

) Unexpectedly, we found no significant predation

m

3

-

10 1 ×

m

n

× on herring larvae within Greifswald Bay. We con-

n clude that in these types of nursery grounds, 1 0.1

N larval mortality due to predation is negligible and

abundance ( abundance

abundance ( abundance that observations from offshore areas should not 0.1 0.01

larvae be transferred. Moreover, low predation rates 10 km stickleback might act synergistically with favorable temper- 0.01 0 Figure 1. Sampling techniques. A) Bongo-net (oblique haul plankton net with Figure 2. Spatial overlap of herring larvae and threespine stickleback (Gasterosteus atures and nutritional conditions for larval growth meshes of 335 & 780 µm) B) Frame trawl with 2.1 m² opening and 5 mm meshes. aculeatus) in May 2011 (calendar week 19) based on the Bongo-net sampling at 36 C) Modified epibenthic sledge, equipped with a plankton net (500 µm) D) Beach stations within Greifswald Bay. Larvae abundances are given as black circles (area = - making transitional waters even more valuable seine with 7 m opening and 5 mm meshes. proportional to abundance) while the stickleback abundance (color scale) was as spawning & nursery grounds for marine fishes. interpolated using a spatial kriging approach.

We gratefully thank all colleagues at the Thünen-Institute and the crew of the research vessel ‘Clupea’. This work resulted from the BONUS BIO-C3 and INSPIRE projects and was supported by BONUS (Art 185), funded jointly by the EU and the Federal Ministry of Education and Research of Germany (BMBF)

Paul Kotterba (corresponding author) Thünen-Institute of Baltic Sea Fisheries Alter Hafen Sued 2 18 069 Rostock, Germany [email protected]