Quantifying predation on Baltic cod early life stages

Abstract

Predation on cod (Gadus morhua) eggs by sprat (Sprattus sprattus) and herring (Clupea harengus) is known to be one of the processes influencing reproductive success of the eastern Baltic cod and has been reported to have contributed to lack of recovery of the stock in the 1990s. This study quantifies the predation on cod eggs in the Bornholm Basin, the major spawning area of cod in the central Baltic Sea, in the 1990s in comparison with the second half of the 2000s. The analyses involve estimating daily consumption rates of predator populations, which are then compared with corresponding daily egg production rates. As a methodological advancement compared with earlier studies, spatially resolved information on predator distribution and abundance is utilized in quantifying predator stock size. This resulted in more realistic consumption estimates in relation to overall egg production compared with earlier studies that consistently overestimated predation pressure by clupeids. Our results suggest a generally lower predation pressure on cod eggs in the mid- to late 2000s, due to a combination of reduced predator abundance and lower daily rations by individual predators.

Fish egg predation by Baltic sprat and herring: do species characteristics and development stage matter?

“…The details on sampling gears, catching methods, and processing of samples are described in Neumann et al (2014). The data and calculation procedures involved in deriving fish egg consumption and production rates are described in Neumann et al (2017). Thus, the material and methods section here focuses on describing the data and analyses specific to this paper, while for description of the data and analyses performed in early studies and further utilized in this paper, the reader is referred to these earlier investigations.…”

“…Ichthyoplankton samples were collected with a Bongo-net with 335 µm mesh size on a station grid of 10 x 12.5 nm in the central Bornholm Basin in the Baltic Sea (see Neumann et al 2017 for further details). The grid consisted of ~45 evenly spaced stations on which double-oblique Bongo hauls that sampled the entire water column down to ca 5 m above the seafloor were conducted.…”

“…The estimation procedures are described in Neumann et al (2017), here deployed for both cod and sprat egg consumption and on stage specific basis.…”

“…The egg production was determined for cod eggs as described by Neumann et al (2017), but additionally considering each egg stage separately. This implied applying temperature dependent stage durations and stage specific egg mortality rates, the latter averaged for each sampling date weighted by the stage durations and finally averaged over all sampling dates.…”

“…In the Baltic Sea, a preference for older egg stages would imply that the impact of egg predation on recruitment of cod (Köster and Möllmann 2000a;Neumann et al 2017) and sprat (Köster and Möllmann 2000b) may be severe, if predation mortality is mainly acting on the egg stages, which have survived the often detrimental hydrographic conditions in the central Baltic Sea (Wieland et al 1994). …”