Baltic cod recruitment – the impact of climate variability on key processes

Abstract

Large-scale climatic conditions prevailing over the central Baltic Sea resulted in declining salinity and oxygen concentrations in spawning areas of the eastern Baltic cod stock. These changes in hydrography reduced the reproductive success and, combined with high fishing pressure, caused a decline of the stock to the lowest level on record in the early 1990s. The present study aims at disentangling the interactions between reproductive effort and hydrographic forcing leading to variable recruitment. Based on identified key processes, stock dynamics is explained using updated environmental and life stage-specific abundance and production time-series. Declining salinities and oxygen concentrations caused high egg mortalities and indirectly increased egg predation by clupeid fish. Low recruitment, despite enhanced hydrographic conditions for egg survival in the mid-1990s, was due to food limitation for larvae, caused by the decline in the abundance of the copepod Pseudocalanus sp. The case of the eastern Baltic cod stock exemplifies the multitude effects climatic variability may have on a fish stock and underscores the importance of knowledge of these processes for understanding stock dynamics.

INCREASEDCONTRACAECUM OSCULATUMINFECTION IN BALTIC COD (GADUS MORHUA) LIVERS (1982–2012) ASSOCIATED WITH INCREASING GREY SEAL (HALICHOERUS GRYPHUS) POPULATIONS

“…The Baltic cod population has, like the grey seal population, fluctuated over the last 30 yr and has only recently recovered from the historically low stock observed in the early 1990s (Kö ster et al 2005;Marine Stewardship Council 2011). Numerous abiotic factors influence spawning success and egg survival of Baltic cod Hü ssy et al 2012) and it cannot be excluded that these parameters have had an influence on our results.…”

An empirical model of the Baltic Sea reveals the importance of social dynamics for ecological regime shifts

“…Important ecosystem dynamics were included by modeling biomasses of sprat (Sprattus sprattus), herring (Clupea harengus) and the zooplankton species Pseudocalanus acuspes (hereafter "zooplankton"), in addition to cod (Gadus morhua). Sprat and herring were important prey species for cod, as was zooplankton for sprat, herring, and cod larvae (20,37). Biomasses of sprat and zooplankton displayed rapid changes at a similar time to the cod collapse (38) and are believed to have been involved in a prey-to-predator loop that encouraged a low-cod, high-sprat state after the collapse (7).…”

“…In a significant advance for natural resource modeling, and for social-ecological modeling more generally, use of the generalized modeling approach (17,18) enabled us to empirically parameterize, dynamically model, and analyze the qualitative social and ecological dynamics of the Baltic cod fishery at comparable levels of detail and without detailed specification of causal relationships. The Baltic cod fishery was selected because the ecological dynamics during the cod boom and collapse have been well-studied (10,19,20), and information about fisher behavior and institutional settings, such as regulation and subsidy policy, is available. Additionally, the cod boom and collapse are qualitatively distinct features of the socialecological system's dynamics that are amenable to the concepts and methods of dynamical systems theory (21), such as stability.…”

Decline and Recovery of Atlantic Cod (Gadus morhua) Stocks throughout the North Atlantic

“…The mechanisms by which pre-recruit survival is affected involve many aspects of the physical and biotic environment. Physical mechanisms include annual variability in the transport of the zooplanktonic food for cod larvae into the Barents Sea and North Sea (Sundby 2000) and the replenishment of salinity and oxygen in the Baltic Sea by episodic inflow of water from the Kattegat and the North Sea (Köster et al 2005). Some examples involving feeding and predation are discussed below.…”