Introduction 362Determination of populations and management units 363 Mismatch between management and biological units in the NE Atlantic 371Fisheries management and management units in the NE Atlantic 371Population structure of demersal fish species 372Cod 372Haddock 378Other demersal roundfish species 378Plaice 379Sole 379Other flatfish species 380 AbstractAn essential prerequisite of a sustainable fisheries management is the matching of biologically relevant processes and management action. In fisheries management and assessment, fish stocks are the fundamental biological unit, but the reasoning for the operational management unit is often indistinct and mismatches between the biology and the management action frequently occur. Despite the plethora of population genetic data on marine fishes, to date little or no use is made of the information, despite the fact that the detection of genetic differentiation may indicate reproductively distinct populations. Here, we discuss key aspects of genetic population differentiation in the context of their importance for fisheries management. Furthermore, we evaluate the population structure of all 32 managed marine fish species in the north-east Atlantic and relate this structure to current management units and practice. Although a large number of studies on genetic population structure have been published in the last decades, data are still rare for most exploited species. The mismatch between genetic population structure and the current management units found for six species (Gadus morhua, Melanogrammus aeglefinus, Merlangius merlangus, Micromesistius poutassou, Merluccius merluccius and Clupea harengus), emphasizes the need for a revision of these units and questions the appropriateness of current management measures. The implementation of complex and dynamic population structures into novel and less static management procedures should be a primary task for future fisheries management approaches.

Recent advances in the application of stock identification methods have revealed inconsistencies between the spatial structure of biological populations and the definition of stock units used in assessment and management. From a fisheries management perspective, stocks are typically assumed to be discrete units with homogeneous vital rates that can be exploited independently of each other. However, the unit stock assumption is often violated leading to spatial mismatches that can bias stock assessment and impede sustainable fisheries management. The primary ecological concern is the potential for overexploitation of unique spawning components, which can lead to loss of productivity and reduced biodiversity along with destabilization of local and regional stock dynamics. Furthermore, ignoring complex population structure and stock connectivity can lead to misperception of the magnitude of fish productivity, which can translate to suboptimal utilization of the resource. We describe approaches that are currently being applied to improve the assessment and management process for marine fish in situations where complex spatial structure has led to an observed mismatch between the scale of biological populations and spatially-defined stock units. The approaches include: (i) status quo management, (ii) “weakest link” management, (iii) spatial and temporal closures, (iv) stock composition analysis, and (v) alteration of stock boundaries. We highlight case studies in the North Atlantic that illustrate each approach and synthesize the lessons learned from these real-world applications. Alignment of biological and management units requires continual monitoring through the application of stock identification methods in conjunction with responsive management to preserve biocomplexity and the natural stability and resilience of fish species.

Dickey-Collas, M., Nash, R. D. M., Brunel, T., van Damme, C. J. G., Marshall, C. T., Payne, M. R., Corten, A., Geffen, A. J., Peck, M. A., Hatfield, E. M. C., Hintzen, N. T., Enberg, K., Kell, L. T., and Simmonds, E. J. 2010. Lessons learned from stock collapse and recovery of North Sea herring: a review. – ICES Journal of Marine Science, 67: 1875–1886. The collapse and recovery of North Sea herring in the latter half of the 20th century had both ecological and economic consequences. We review the effect of the collapse and investigate whether the increased understanding about the biology, ecology, and stock dynamics gained in the past three decades can aid management to prevent further collapses and improve projections of recovery. Recruitment adds the most uncertainty to estimates of future yield and the potential to reach biomass reference points within a specified time-frame. Stock–recruitment relationships must be viewed as being fluid and dependent on ecosystem change. Likewise, predation mortality changes over time. Management aimed at maximum sustainable yield (MSY) fishing mortality targets implies interannual variation in TACs, and variability in supply is therefore unavoidable. Harvest control rules, when adhered to, aid management greatly. We advocate that well-founded science can substantially contribute to management through improved confidence and increased transparency. At present, we cannot predict the effects of collapse or recovery of a single stock on the ecosystem as a whole. Moreover, as managers try to reconcile commitments to single-species MSY targets with the ecosystem-based approach, they must consider the appropriate management objectives for the North Sea ecosystem as a whole.

Transport of plaice eggs and larvae in the southern North Sea was modelled using a finite-volume advection-dispersion model with high spatial and temporal resolution. Hydrodynamic forcing was based on actual river discharge and meteorological data for a 7 mo period in the winter and spring of 1989 and 1996 to 2003. Egg and larval development was linked to temperature and each developmental stage had its own physical and behavioural properties. Two hypotheses on transport mechanisms in the late larval stages were compared: passive drift versus selective tidal stream transport (triggered by environmental cues). Modelled larval stage durations closely corresponded to empirical estimates based on otolith daily ring counts. Close correspondence was also observed between model results and survey data on settlement in the western Wadden Sea, especially for the abundance of the strong 1996 year-class and the early settlement of the 2002 year-class. Transport patterns and the proportion of larvae reaching inshore nurseries varied substantially between years. Recruitment was negatively correlated with the distance over which eggs and larvae were transported, probably because of a breakdown in connectivity between spawning and nursery areas with increased transport rates. In conclusion, meteorologically driven variability in the hydrodynamics of the southern North Sea greatly affects the transport patterns of plaice eggs and larvae and may play an important role in determining recruitment variability and spatial connectivity. KEY WORDS: Biophysical model · Hydrodynamic transport · Larval behaviour · Interannual variability · Connectivity · Recruitment · Pleuronectes platessa · North SeaResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 390: 195-211, 2009 important nursery area for North Sea plaice (Zijlstra 1972), density-dependent mortality only fine-tunes recruitment variability, and the main factors driving variations in recruitment occur prior to the juvenile life-phase (van der Veer 1986, van der Veer et al. 2000.Several hypotheses have been postulated on the factors causing recruitment variability during the egg and larval stages. Cold winters often produce strong yearclasses (van der Veer & Witte 1999, Fox et al. 2000, and low egg mortality is associated with low seawater temperatures (Harding et al. 1978, Dickey-Collas et al. 2003. These low temperatures may also reduce larval mortality due to lower predator activity or to an improved match between the production cycles of larvae and their food (Cushing 1972, Leggett & Deblois 1994; however, this may be offset by slower development of the larvae (Pepin 1991). Finally, variations in transport of larvae to coastal nursery grounds may contribute to variations in recruitment success (Nielsen et al. 1998, van der Veer et al. 1998.In the present study we examined interannual variability in plaice egg and larval transport in the southern North Sea. We focused on the role of transport in determining recruitment var...