Eastern Baltic cod:perspectives from existing data on processes affecting growth and survival of eggs and larvae

Abstract

ABSTRACT. We review eastern Baltic cod literature to identify areas for future research involving egg and larval stages. Egg and larval abundances have been estimated for several decades in all of the major spawning areas. Using a subset of the available data we show that the timing of peak egg abundance in one of these areas (Bornholm Basln) vanes seasonally by at least 2 mo. Trends in egg and larval abundance over time are not obvious due to differences In gear type, sampling intensity, and survey timing relative to spawning dates Interannual differences in larval transport away from spawning areas may also contribute to variability in abundance estimates The results of broad-scale zooplankton surveys suggest that the abundance and types of potential prey in the Baltic Sea are similar to those in other regions and perhaps suitable for moderate-to-fast larval growth However, few systematic surveys have described food concentrations at appropriate scales for cod larvae, and the species composition of larval diets is unknown. Growth rates for Baltic cod larvae have not been measured and cannot be compared with rates in other areas or to vanatlons in biotic and ab~otlc factors. Large gaps exist in our knowledge of processes affecting egg and larval growth and survival, but some promising areas of research are indicated. In particular the seasonality of spawning, deep water oxygen concentrations, predation on eggs, and larval food production require further investigation

Developing Baltic cod recruitment models. II. Incorporation of environmental variability and species interaction

“…The hydrographic regime in 1994 was characterised by higher than average salinities and, in the beginning of the spawning season, high oxygen concentrations in and below the halocline. However, as the spawning peak was late in the year, eggs were nevertheless exposed to relatively low oxygen levels (MacKenzie et al 1996), thereby resulting in oxygen-related survival rates similar to those in 1995 and 1996 (54-60%). Egg predation by sprat was lower from 1993 on than in preceding years and was caused by (i) a continuous shift in spawning time of cod to a later month , resulting in a limited temporal overlap between sprat as predators and cod eggs as prey (Köster and Möllmann 2000a) and (ii) a limited vertical overlap between predator and prey due to the changed hydrographic regime (Köster and Möllmann 2000a).…”

“…First, we would like to emphasise that processes coupling individual growth, sexual and gonadal maturation, as well as quantity and quality of produced spawning products to environmental conditions (e.g., Marshall and Frank 1999;Trippel 1998) are a promising field of research in this respect. Furthermore, anthropogenic impacts, such as contamination by toxic substances, potentially affecting the viability of spawning products and eutrophication and impacting the oxygen consumption rates in deep Baltic basins and thus the ambient oxygen conditions for cod egg development (e.g., MacKenzie et al 1996) have to be verified and incorporated when projecting over longer time periods. Finally, the importance of structural uncertainty in medium-to long-term projection models, i.e., which variables to incorporate and how to incorporate them into the model (e.g., linear, exponential, additive), remains an area for further development; a key issue in the development of quantitative relationships between environmental variables and fish populations as interactions may be counterintuitive (Rose 2000 Table A2.…”

“…Mean annual egg and larval abundance values are available for the different subdivisions from ichthyoplankton surveys in (Appendix Tables A1 and A2) based on data compiled by Karasiova (1995), E.M. Karasiova (Atlantic Scientific Research Institute of Marine Fisheries and Oceanography, Kaliningrad, Russia, personal communication), MacKenzie et al (1996), Makarchouk (1997), A. Makarchouk (Latvian Fisheries Research Institute, Riga, Latvia, personal communication), R. Voss (Institute of Marine Sciences, Kiel, Germany, personal communication), and Wieland et al (2000). Egg abundance estimates are based on a 3-month period encompassing the main spawning period, i.e., AprilJune up to 1989 and successively shifted to May-June (1990 and June-August (1993 to take into account a shift in the spawning period in recent years .…”

“…In the last 10-15 years, it has been possible to examine many of the major processes currently believed to affect recruitment variability of the cod (Gadus morhua) in the Central Baltic (e.g., Bagge and Thurow 1994;MacKenzie et al 1996). As a result, several new time series are becoming available for variables relating to these processes.…”

Scientific knowledge of biological processes potentially useful in fish stock predictions

“…For example, early spawned cod eggs in the Baltic encounter on average better oxygen conditions than later spawned ones, due to a continued oxygen consumption in the water layers of egg development after inflows of oxygenated water at the beginning of the year (MacKenzie et al, 1996). On the other hand, early spawned cod eggs will experience higher predation pressure, due to increased temporal and spatial overlap between sprat as predators and eggs as prey (Köster and Möllmann, 2000a).…”