The North Atlantic spring-bloom system - where the changing climate meets the winter dark

-The North Atlantic with its spring-bloom ecosystem has its particular responses to climate change, many of them different from the other parts of the world's oceans. The system is strongly influenced by anthropogenic climate change as well as to strong decadal to multidecadal natural climate variability. In particular, the northernmost part of the system and the Arctic is exposed to higher increase in temperature than any other ocean region. The most pronounced examples of poleward migration of marine species are found in the North Atlantic, and comprise the recent warming phase after the 1970s. The latitudinal asymmetric position of the Arctic Front and its nature of change result in a considerably larger migration distance and migration speed of species in the Northeast Atlantic part of the system. However, we here hypothesize that there is a limit to the future extent of poleward migration of species constrained by the latitudinal region adjacent the Polar Circle. We define this region the critical latitudes. This is because the seasonal light cycle at high latitudes sets particular demands on the life cycle of planktivorous species. Presently, boreal planktivorous species at high latitudes deposit lipids during the short spring bloom period and overwinter when phytoplankton production is insufficient for feeding. Unless invading temperate species from farther south are able to adapt by developing a similar life cycle future poleward migration of such species will be unlikely

Management of transboundary and straddling fish stocks in the Northeast Atlantic in view of climate-induced shifts in spatial distribution

The introduction of 200 n.m. exclusive economic zones (EEZs) in the late 1970s required increased collaboration among neighbouring coastal states to manage transboundary and straddling fish stocks. The established agreements ranged from bilateral to multilateral, including high‐seas components, as appropriate. However, the 1982 United Nations Convention on the Law of the Sea does not specify how quotas of stocks crossing EEZs should be allocated, nor was it written for topical scenarios, such as climate change with poleward distribution shifts that differ across species. The productive Northeast Atlantic is a hot spot for such shifts, implying that scientific knowledge about zonal distribution is crucial in quota negotiations. This diverges from earlier, although still valid, agreements that were predominately based on political decisions or historical distribution of catches. The bilateral allocations for Barents Sea and North Sea cod remain robust after 40 years, but the management situation for widely distributed stocks, as Northeast Atlantic mackerel and Norwegian spring‐spawning herring, appears challenging, with no recent overall agreements. Contrarily, quotas of Northern hake are, so far, unilaterally set by the EU despite the stock's expansion beyond EU waters into the northern North Sea. Negotiations following the introduction of EEZs were undertaken at the end of the last cooler Atlantic Multidecadal Oscillation (AMO) period, that is, with stock distributions generally in a southerly mode. Hence, today's lack of management consensus for several widely distributed fish stocks typically relates to more northerly distributions attributed to the global anthropogenic signal accelerating the spatial effect of the current warmer AMO.publishedVersio

Stereological calibration of the profile method to quickly estimate atresia levels in fish

Journal homepage: http://www.elsevier.com/wps/find/journaldescription.cws_home/503309/description#descriptionThe (physical) ‘disector method’, a frequently cited stereological technique, has so far received little attention within fish fecundity studies, although it can be used to provide unbiased, assumption-free data on levels of atresia (resorption of vitellogenic oocytes). In comparison, traditional simple counting to estimate the numerical ratio of normal to atretic cells is biased because the smaller atretic cells have a lower chance of being transected in histological sections. These problems are circumvented by the disector method as it operates in three dimensions, i.e., uses consecutive pairs of slides. However, the high labour costs involved prevent regular usage of this method in population studies where large numbers of ovarian samples are processed. In the present paper we assess the corresponding bias of the traditional profile method, analysing developing ovaries of Atlantic cod (Gadus morhua) and herring (Clupea harengus), i.e., in a relatively broad and narrow oocyte frequency situation, respectively. A highly significant but non-linear relationship (r2 = 0.975, P < 0.001, df = 154) was found between the relative intensity of atretic vitellogenic oocytes (ARI) estimated by the disector and profile method. Both species fitted well to this polynomial model. The degree of underestimation of atretic oocytes still containing yolk (i.e., the alpha stage) was at maximum (11.6%) at intermediate levels (ARI = 50% (disector)) but, logically, no such bias existed at the extreme ends (ARI: 0% and 100% (disector)). The practical use of this simple, fast approach designated the stereo-profile method, including the additional use of image analysis for further refinements, is successfully demonstrated on field samples

Half a century of high-latitude fisheries oceanography research on the "recruitment problem" in Northeast Arctic cod (Gadus morhua)

Predicting recruitment in a reliable fashion is a great challenge within operational fisheries advice. Here, we consulted the unique but undercommunicated IMR Cod Larva Project (1975–1990), its spin-offs, placed in an international era of advancements over the last 50 years to glance into the future. Few initiatives of this kind have applied such extensive research approaches, spanning from laboratory, mesocosm, tank, and field studies to process modelling. The “critical period” concept appeared misleading, covering months rather than days of the early life history stages (ELHS) of Northeast Arctic cod. Larval feeding success was strongly modified by improved encounter rates from wind-induced turbulence. In addition, the following maternal effect studies evidenced that the dynamics of stock demography prior to spawning should be upheld to promote recruitment success. Although we now have lower-trophic level models as well as ELHS individual-based models, such models are still insufficiently reflecting the needed spatiotemporal resolution. The same problem applies to climate/circulation models. Nevertheless, this long-lasting research has significantly improved the mechanistic understanding of ELHS dynamics but also of the more predictable adult reproductive parameters. Based on a “to-list list,” we suggest research avenues that should be pursued to further improve our ability predicting recruitment strength in marine fish stocks.publishedVersio

Systematic bias in estimates of reproductive potential of cod stocks: implications for stock/recruit theory and management

Stock/recruit relationships, describing the relationship between the parental population and the number of offspring produced, are a central tool in population ecology. For fish populations the stock/recruit relationship uses spawning stock biomass (SSB) to represent reproductive potential of the parental population. This assumes that the proportion of SSB comprised of females and the relative fecundity (number of eggs produced per unit mass) are both constant over time. To test these two constancy assumptions female-only spawner biomass (FSB) and total egg production (TEP) were estimated for the Northeast Arctic cod stock over a 56-year time period (1946-2001). During that time period the proportion of females (FSB/SSB) varied between 24 and 68% and the variation was systematic with length such that SSB became more female-biased as the mean length of spawners increased and more male-biased as mean length decreased. Over the same time period, relative fecundity of the stock (TEP/SSB) varied between 115 and 355 eggs g-1 and, like FSB/SSB, was significantly, positively correlated mean length of spawners. Because both FSB/SSB and TEP/SSB covaried with length composition, SSB is systematically biased estimate of reproductive potential. FSB and TEP were evaluated as possible replacements for SSB in stock/recruit relationship. Both indices gave a different interpretation of the recruitment response to reductions in stock size (over-compensatory) compared to that obtained using SSB (either compensatory or depensatory). The threshold level of stock size below which recruitment becomes impaired was estimated for each of the different stock/recruit relationships using piecewise linear regression. There was no difference between SSB and FSB in the assessment of stock status, however, in recent years (1980-2001) TEP fell below the threshold level more frequently than SSB fell below. This suggests that using SSB as a measure of stock reproductive potential may lead to overly optimistic assessments of stock status

Drift indices confirm that rapid larval displacement is essential for recruitment success in high-latitude oceans

Larval drift is a key process for successful fish recruitment. We used Norwegian spring-spawning herring (Clupea harengus) as model species to investigate the relationship between larval drift and recruitment. Larval drift indices were derived from simulations based on survey observations between 1993 and 2016. We show that forward simulated larval drift indices have an important positive relation to recruitment success. The relationship demonstrates elevated recruitment when larvae relocate rapidly northwards toward the Barents Sea. Negative or low larval drift indices coincide with only weak recruitment emphasizing limited survival in years with enhanced larval retention. Hence, with this work we combine drift model outcomes refined with survey data indicating that more extensive larval drift is an important component in population dynamics for high-latitude small pelagic fishes. However, larval displacement alone represents only one among many controlling factors but may offer possible predictions of the probability of higher or lower recruitment in the short term. The applicability of the drift indices is adaptable in all world oceans and all marine organisms that occupy planktonic life stages exposed to dynamic ocean currents. The study demonstrates how larval drift indices help to identify larval transport or retention to be crucial for population replenishment.publishedVersio

Egg buoyancy variability in local populations of Atlantic cod (Gadus morhua)

http://dx.doi.org/10.1007/s00227-012-1984-8Previous studies have found strong evidences for Atlantic cod (Gadus morhua) egg retention in fjords, which are caused by the combination of vertical salinity structure, estuarine circulation, and egg specific gravity, supporting small-scaled geographical differentiations of local populations. Here, we assess the variability in egg specific gravity for selected local populations of this species, that is, two fjord-spawning populations and one coastal-spawning population from Northern Norway (66–71°N/10–25°E). Eggs were naturally spawned by raised broodstocks (March to April 2009), and egg specific gravity was measured by a density-gradient column. The phenotype of egg specific gravity was similar among the three local populations. However, the associated variability was greater at the individual level than at the population level. The noted gradual decrease in specific gravity from gastrulation to hatching with an increase just before hatching could be a generic pattern in pelagic marine fish eggs. This study provides needed input to adequately understand and model fish egg dispersal

Mechanisms regulating oocyte recruitment and skippedspawning in Northeast Arctic cod (Gadus morhua L.)

Journal homepage: http://pubs.nrc-cnrc.gc.ca/rp-ps/journalDetail.jsp?jcode=cjfas&lang=engTo examine mechanisms that affect fecundity, atresia and skipped spawning in Northeast Arctic cod (Gadus morhua L.), we conducted an experiment where wild-caught cod (>60 cm) kept under restricted food regimes were subjected to monthly biopsies, hormonal and physical measurements. The power of body weight as a fecundity proxy increased until the presumed end of follicle proliferation in early November, thereafter it remained stable. Atresia occurred in most females; but, for maturing females, mainly close to spawning. 18 % of the females had small gonads with predominantly previtellogenic oocytes at sacrifice in January. These females were past-spawners, verified by post ovulatory follicles in their gonadss. These „skippers‟ had lower condition than maturing cod from December, smaller livers upon sacrifice and lower plasma 17-β estradiol values from early November. Until November, oocytes developed similarly for all females, but in November oocyte development was arrested at the early cortical alveoli stage and atresia occurred in all skippers. In sum, fecundity and skipped spawning seem highly influenced by energy reserves during early vitellogenesis and limited to females only. Finally, skippers were identifiable long before the predicted onset of spawning, which could have implications for forecasting of egg production and hence stock-recruitment relationships

From “where” and “when” to “what” and “why”: archival tags for monitoring “complex” behaviours in fish.

Understanding the movements (“where” and “when”) and behaviour (“what” and, hopefully, “why”) of individuals and populations is key to answering fundamental questions in fish ecology. The use of archival tags in telemetry studies of marine fish have, by and large, involved recording “simple” measurements of variables such as pressure (giving depth), temperature and light over extended timescales. These have then been used to provide information about location and movement of individuals. However, our understanding of more complex behaviours (i.e. what fish are doing as different from spatial movements) has usually been inferred from movement data because it has not been possible to record directly specific behavioural events such as feeding or spawning. This is because the events are usually infrequent, irregular and often quite brief and so not amenable to a technology based on taking regular but infrequent records of continuously available variables. The recent implementation of new sensors (e.g. physical movement, tri-axial accelerometers), rapid (< 30 Hz) sampling capabilities, enhanced memory and more complex data capture protocols has lead to the development of archival tags that can be used to detect and record complex behaviours such as feeding and spawning. We describe recent developments with archival tags and their use to monitor feeding and spawning in fish together with the application of tri-axial accelerometry that can be used to quantify behaviour and metabolic rate. These can then be used to assess the cost of behaviours with a view to understanding how appropriate they are as responses to environmental variability. Keywords: telemetry, behaviour, data storage ta

Linking the dynamic organization of the ovary with spawning dynamics in pelagic fishes

The link between the dynamic organization of the ovary and spawning dynamics was investigated in pelagic fishes with indeterminate fecundity. Αn array of laboratory methods and statistical approaches was applied to samples of gonadal material from three commercial NE Atlantic fish species: the sardine, Sardina pilchardus, the horse mackerel, Trachurus trachurus, and the mackerel, Scomber scombrus. Methods included the application of clustering analysis algorithms in histological specimens and the application of particle analysis on whole mounts. More specifically, various attributes of ovarian dynamics such as the oocyte size frequency distribution, the ratio of total to batch fecundity, the number of oocyte cohorts, and the recruitment of early secondary growth oocytes were related to historic estimates of spawning interval and oocyte growth rate. It was shown that indeterminate spawners can display varying proportions of oocytes at the size range between primary and secondary growth based on the seasonal pattern of oocyte recruitment. This finding indicates that determinacy and indeterminacy should rather be recognized as end-points along a continuum which is controlled by the degree of overlap between oocyte recruitment period and spawning period. It was also demonstrated that fishes like sardine with long spawning intervals and fast oocyte growth exhibit relatively few, clearly separated oocyte cohorts, while fishes like mackerel with shorter spawning interval display increased number of coexisting cohorts. Ultimately, these aspects may provide a proxy of spawning interval and thereby spawning frequency which is a variable of paramount importance in biomass assessments of commercial fish stocks through egg production methods.Versión del editor2,01