Description: Besides variable egg survival, previous studies suggested that the larval stage may be the most critical phase in determining Baltic cod recruitment variability, and that larvae need to conduct an ontogenetic vertical migration from hatching depths (〉50 m) to upper layers with increased food availability in order to initiate first feeding, improve their nutritional condition and growth, and avoid starvation. Recently, detailed information on the stage-resolved vertical distribution of main Baltic copepod species, including the preferred larval Baltic cod prey species Pseudocalanus acuspes, has become available. Therefore, the vertical distribution of Baltic cod larvae in August 2007 and their depth-dependent nutritional condition and growth were investigated. RNA–DNA based methods were used to estimate growth, including a novel approach to estimate growth performance by relating observed specific growth rates (SGR) of field caught larvae to temperature-dependent reference growth rates (Gref) for fast-growing laboratory reared fish from the literature. This standardization to Gref was found to have a great potential to improve investigations on the growth and ecology of larval fish. The need for early larvae to migrate to shallower layers was corroborated, while larger size classes were found at increasingly greater depths. This may reflect a continuation of the ontogenetic vertical migration in order to follow increasingly larger prey items at greater depths and to save energy in cooler waters below the thermocline. Larval growth generally declined with increasing depth, but the decline in growth became less pronounced in larger size classes. This indicates that larger larvae were better in coping with the ambient environment and the available prey field at greater depths. Generally, Baltic cod larvae grew poorly compared to larvae from other studies, which is discussed in relation to differences in predation and a possible food–temperature trade-off for larvae in the highly stratified Baltic Sea. Comparison with earlier results showed a higher frequency of starving larvae and lower frequencies of larger larvae after the first-feeding stage in 1994 and 1995. As this was a period of low Baltic cod recruitment despite favourable conditions for egg survival, it is concluded that larval starvation mortality has a high potential to contribute to recruitment variability in Baltic cod. Highlights ► We investigate the vertical distribution and depth-dependent growth of Baltic cod larvae. ► We apply a novel approach which relates observed growth rates to reference growth rates in order to obtain growth performance. ► Assessing growth performance was found to be a valuable tool in studies on the growth and ecology of larval fish. ► The need for an ontogenetic first-feeding migration was corroborated and growth generally declined with increasing depth. ► Baltic cod larvae were generally growing badly, suggesting starvation-induced recruitment regulation during the larval stage.

Description: At present, several cod stocks are outside safe biological limits and are managed under recovery plans. For these stocks Total Allowable Catches (TAC's) are generally low and quotas are accompanied by a broad variety of technical measures influencing the fishing patterns. Consequently, the input data to stock assessment models relying on catch statistics from the commercial fisheries is potentially biased and the perception of stock status may be incorrect. Egg production methods (EPM) provide a fishery independent alternative. Additionally, they provide better estimates of stock reproductive potential (SRP). Eastern Baltic cod (Gadus morhua callarias L.) has severely declined throughout the 2nd half of the 1980s and 1st half of the 1990s due to climate-driven adverse hydrographic conditions and high fishing intensity. Since 2007 the stock is managed under a long-term management plan and showed signs of recovery in most recent years. Since 1986, egg surveys have been carried out regularly in the Bornholm Basin, the most important spawning area of Eastern Baltic cod since mid-1980s. In the present paper the robustness of EPM towards simplification of spawning parameters and towards reduction of the number of egg surveys is tested applying three different methods requiring different numbers of egg surveys. We applied the annual egg production method (AEPM) requiring full egg survey coverage of the spawning season to estimate cod abundances in the Bornholm Basin. In addition, the daily fecundity reduction method (DFRM) and the daily egg production method (DEPM) were tested, the latter two methods requiring only single egg surveys, but require more complex reproduction input parameters. All three methods provided a comparable result, which was also expected as many spawning parameters were derived from the same underlying data sets. In a sensitivity analysis several input parameters were varied simultaneously up to 20% in both directions. EPM were especially sensitive towards changes in proportions of mature females at age, whereas changes in the various fecundity parameters and spawning fraction were less influential. EPM results followed the large scale spawning stock trends of the Baltic International Trawl Survey index, whereas the year to year variations of the index were not captured to well. EPM yielded spawning stock sizes in the same order of magnitude as provided by a spatially down-scaled multi-species stock assessment model.

Description: Highlights: • Egg production methods have been used for assessing the Eastern Baltic cod stock. • The analysis contributed to the re-establishment of an analytical assessment for the stock. • Annual and daily egg production methods gave similar results. • Results confirmed trends from trawl surveys, questioned earlier because of potential change in catchability. • Several biological processes impact on absolute spawning stock estimates and research needs are identified to improve these. Abstract: Egg production methods (EPM) provide fishery independent estimates of spawning stock sizes and dynamics of fish populations. Such methods are commonly used for short-lived pelagic species, less so for demersal fish such as cod. In this paper, we apply EPMs on cod in the eastern Baltic Sea, using a long time series of ichthyoplankton data. Stock assessment of Eastern Baltic cod has been challenged due to changing productivity of the stock invalidating some of the standard procedures, e.g. age determination and input variables, e.g. natural mortality. We demonstrate that EPMs, based on other data and assumptions than standard stock assessments, provide useful information on stock status and dynamics. We apply both the annual and daily egg production methods, which yielded similar results and were in line with stock trends derived from bottom trawl surveys. However, the EPM based spawning stock estimates were consistently lower compared to results from the latest analytical stock assessment. We identified processes introducing uncertainties in EPM estimates and their effects on the resulting estimates, and conclude that they mainly affect the absolute estimates but less the relative trends in stock dynamics. Therefore, we consider that EPMs are useful for providing relative indices for stock assessment purposes, with the Eastern Baltic cod being the first case where such indices are included in an official stock assessment of a demersal gadoid species. We also identify knowledge gaps in order to be able to derive absolute stock size estimates from EPMs in the future.

Description: Highlights: • Designing spawning closures requires consideration of the mechanisms through which the closures can affect the fish stocks. • Small area closures may have unintended negative effects to the stocks due to fishing effort reallocation. • Closures covering most of the stock distribution are more robust to gaps in biological knowledge than small area closures. Abstract: Fisheries management measures often include spatio-temporal closures during the spawning period of the fish with an overarching aim of improving the stock status. The different mechanisms how a spawning closure potentially can influence the stock are often not explicitly considered when designing such closures. In this paper, we review and synthesize the available data and knowledge on potential effects of the implemented spawning closures on cod in the Baltic Sea. The Baltic cod example represents a relatively data rich case, which allows demonstrating how a closure might affect different parameters of stock status via different mechanisms, including potential unintended negative effects. We conclude that designing relatively small area closures appropriately is highly complex and data demanding, and may involve tradeoffs between positive and negative impacts on the stock. Seasonal closures covering most of the stock distribution during the spawning time are more robust to data limitations, and less likely to be counterproductive if suboptimally designed.

Description: Retention or dispersion of larvae from the spawning ground has been identified as one of the key processes influencing recruitment success in fish stocks. An exercise combining 3-D hydrodynamic model simulations and field data on spatial distributions of juvenile Baltic cod was utilised to investigate the potential drift of larvae from the centre of main spawning effort in the Bornholm Basin, Baltic Sea. In the simulations cod larvae were represented as Lagrangian drifters. Habitats in which larvae and juvenile cod potentially dwell and where juveniles settle were identified to ascertain the importance of predicting transport. The transport of Baltic cod larvae was investigated by detailed drift model simulations for the years 1986 to 1999. The results yielded a clear dependency on wind-induced drift of larval cod, which is mainly controlled by the local atmospheric conditions over the Baltic Sea. Seasonally averaged distributions of drifters were compared with actual distributions of 0-group cod, as determined from bottom and pelagic trawl surveys conducted in autumn of the years 1993 to 2000 in and around the Bornholm Basin. The results suggest that juveniles caught in different areas can be assigned to different times of the spawning season. Because of seasonal differences in the circulation patterns, the southern coastal environment is on average most important for early and late spawners, whereas larvae hatching in mid-summer were on average transported towards the north or to a higher degree remained in the spawning ground.

Description: The feeding habits of co-occurring Baltic sprat (Sprattus sprattus) and cod (Gadus morhua) larvae have been studied in the Bornholm Basin, Baltic Sea during 12 cruises covering the spawning seasons in 1987 and 1988. The seasonal and size-dependant diet composition is described based on Bongo-net samples. Contrary to investigations from other areas, first-feeding larvae of both species included almost no phytoplankton in their diet. Feeding started on calanoid copepod nauplii which were the dominating food item. Copepodite stages I–V and finally adult copepods were eaten with increasing larval length. Only sprat larvae used cladocerans additionally as food source of considerable importance. Cod larvae included copepodites/copepods in their diet at smaller total lengths than sprat larvae. The trophic niche breadth of both species did not increase with larval length. The feeding selectivity of different sized sprat and cod larvae (Pearre’s C-index) was calculated based on vertical resolving sampling of predator and prey. The results indicate a strong preference of sprat larvae for different developmental stages of Acartia spp., a species showing a pronounced increase in standing stock since the 1980s. Cod larvae selected Pseudocalanus elongatus, a species which decreased in biomass. Possible implications for recruitment levels of both species are discussed.

Description: The sprat (Sprattus sprattus balticus S.) population size in the Bornholm Basin, one of the major spawning areas of the species in the Baltic Sea, was estimated with the daily egg production method. Egg abundance, stock structure, sexual and gonadal maturation, spawning frequency and the batch fecundity of sprat throughout the sprat spawning season were simultaneously investigated to obtain an estimate of the population size at spawning time. The results confirmed the population estimate from a hydroacoustic survey, but contrasted spatially down-scaled results from an area dis-aggregated stock assessment model applying fisheries statistics. Conflicting results from both latter methods have previously hampered quantitative studies on recruitment processes of sprat and cod, for example the estimation of predation pressure on cod eggs by sprat. The egg production method did not allow an estimation of the population size of sprat in the entire assessment area larger than the Bornholm Basin, i.e. ICES sub-division 25. This failure is caused by sprat spawning activity outside the Bornholm Basin, not covered by the standard egg surveys and has consequences for the general applicability of available egg abundance time series to retrospectively estimate sprat stock development.