Marine aggregates of biogenic origin, known as marine snow, are considered to play a major role in the ocean's particle flux and may represent a concentrated food source for zooplankton. However, observing the marine snow−zooplankton interaction in the field is difficult since conventional net sampling does not collect marine snow quantitatively and cannot resolve so-called thin layers in which this interaction occurs. Hence, field evidence for the importance of the marine snow−zooplankton link is scarce. Here we employed a Video Plankton Recorder (VPR) to quantify small-scale (metres) vertical distribution patterns of fragile marine snow aggregates and zooplankton in the Baltic Sea during late spring 2002. By using this non-invasive optical sampling technique we recorded a peak in copepod abundance (ca. 18 ind. l −1 ) associated with a pronounced thin layer (50 to 55 m) of marine snow (maximum abundance of 28 particles l −1 ), a feature rarely resolved. We provide indirect evidence of copepods feeding on marine snow by computing a spatial overlap index that indicated a strong positively correlated distribution pattern within the thin layer. Furthermore we recorded images of copepods attached to aggregates and demonstrating feeding behaviour, which also suggests a trophic interaction. Our observations highlight the potential significance of marine snow in marine ecosystems and its potential as a food resource for various trophic levels, from bacteria up to fish.

Genetic data have great potential for improving fisheries management by identifying the fundamental management units—that is, the biological populations—and their mixing. However, so far, the number of practical cases of marine fisheries management using genetics has been limited. Here, we used Atlantic cod in the Baltic Sea to demonstrate the applicability of genetics to a complex management scenario involving mixing of two genetically divergent populations. Specifically, we addressed several assumptions used in the current assessment of the two populations. Through analysis of 483 single nucleotide polymorphisms (SNPs) distributed across the Atlantic cod genome, we confirmed that a model of mechanical mixing, rather than hybridization and introgression, best explained the pattern of genetic differentiation. Thus, the fishery is best monitored as a mixed‐stock fishery. Next, we developed a targeted panel of 39 SNPs with high statistical power for identifying population of origin and analyzed more than 2,000 tissue samples collected between 2011 and 2015 as well as 260 otoliths collected in 2003/2004. These data provided high spatial resolution and allowed us to investigate geographical trends in mixing, to compare patterns for different life stages and to investigate temporal trends in mixing. We found similar geographical trends for the two time points represented by tissue and otolith samples and that a recently implemented geographical management separation of the two populations provided a relatively close match to their distributions. In contrast to the current assumption, we found that patterns of mixing differed between juveniles and adults, a signal likely linked to the different reproductive dynamics of the two populations. Collectively, our data confirm that genetics is an operational tool for complex fisheries management applications. We recommend focussing on developing population assessment models and fisheries management frameworks to capitalize fully on the additional information offered by genetically assisted fisheries monitoring.

We studied the effects of food level changes on otolith and somatic growth in postlarval Baltic sprat reared initially for a period of 11 days under zero, low, and ad libitum feeding conditions. During a subsequent 11 day period, feeding regimes were reversed in half of the low and ad libitum feeding treatments, and starved fish were re-fed ad libitum rations. Somatic growth rates under low and ad libitum food rations ranged between 0.15-0.22 mm day −1 and 0.48-0.63 mm day −1 , respectively, and led to significant differences in length and weight between feeding regimes. Previously starved fish, however, grew only 0.25-0.28 mm day −1 under ad libitum conditions. During the first period, significant linear relationships were found for otolith v. length and v. weight growth across all treatments. After changing feeding regimes, increment widths failed to significantly predict somatic growth for 9 days, after which a significant relationship between otolith and somatic growth became re-established. Recent otolith growth was a good predictor of fish condition after the first, but not after the second period. The results suggest that perturbations in environmental conditions can temporarily decouple otolith from somatic growth in postlarval sprat, which needs to be considered in field studies.

European sea bass (Dicentrarchus labrax) is an important target species for recreational and commercial fisheries. In recent years, the spawning stock biomass has declined markedly in some areas, and strict management measures have been introduced. However, the development of appropriate stock assessment and fisheries management has been hampered by a lack of information on post-release mortality. This study investigated post-release mortality of sea bass captured with common recreational fishing gear under experimental conditions in an aquaculture facility over 10 d. Three experiments investigated: (i) the effects of different bait types; (ii) the impact of prolonged air exposure; and (iii) the impact of deep hooking on post-release mortality. By combining the experimental results with country-specific information on sea bass angling practices, estimates of post-release mortality are provided for the northern sea bass stock. No mortality was observed for sea bass captured on artificial baits. The use of natural baits resulted in a mortality of 13.9% (95% CI = 4.7–29.5%), which was associated with deep hooking, hooking injuries, and prolonged air exposure. The use of artificial baits and short air exposure (≤30 s) increased survival probability, whereas deep hooking resulted in 76.5% (95% CI = 50.0–93.2%) mortality. Depending on country-specific angling practices, post-release mortality estimates ranged from 2.8% to 9.1% (mean = 5.0%, 95% CI = 1.7–14.4%) for northern sea bass. Despite these relatively low mortality estimates, post-release mortality should be considered in stock assessments as its cumulative impact may be high. Moreover, post-release mortality can be reduced by implementing species-specific best practice guidelines.

An analysis of mass (M) and standard length (L S ) data for larval, juvenile and adult sprat (Sprattus sprattus; Clupeidae) revealed marked changes in the allometric scaling factor (b in M¼ aL b s ). For sprat <44 mm L S , b was 5Á0, whereas in larger juveniles and adults, b was c. 3Á4 indicating a relatively protracted metamorphic period for this species. # 2005 The Fisheries Society of the British IslesThe mass (M) and length (L) relationship of early life stages of marine fish species has been the subject of considerable research (Laurence, 1979;Safran, 1992;Pepin, 1995) due to the desire to accurately estimate fish condition. The M and L relationship is typically described using a logged form of an allometric equation:(1)where a and b are constants. The range in b-values (slopes) calculated for larvae of marine fishes is considerably larger (e.g. b ¼ 2Á7 to 4Á2) (Laurence, 1979;Pepin, 1995) than that for later developmental stages where isometric slopes (e.g. b ' 3) are often observed (Fulton, 1911;Dawson, 1965;Coull et al., 1989;Safran, 1992;Koutrakis & Tsikliras, 2003). Therefore, in some fish species the slope of the M and L relationship changes during early development until the adult body form is reached. Intra-specific comparisons of M and L data collected for larvae, juveniles and adults, however, are rare. Consequently, changes in the slope of the M and L relationship during ontogeny remain poorly documented for most fish species. The present study addressed this by comparing the M and L relationship over the complete range in body sizes †Author to whom correspondence should be addressed. Tel.: þ49 4042 838 6602; fax: þ49 4042 838 6618;