The match/mismatch hypothesis and the feeding success of fish larvae in ice-covered southeastern Hudson Bay

We studied the synchronism between the seasonal occurrence of fish larvae and their prey in ice-covered southeastern Hudson Bay, Canada, in spring 1988, 1989 and 1990. Arctic cod #Boreogadus saida and sand lance #Ammodytes sp. larvae hatched several weeks before ice break-up and fed primarily on copepod nauplii. The timing of 50% yolk resorption was the same every year (11 to 18 May for Arctic cod and 5 to 11 June for sand lance) but the availability of copepod nauplii varied substantially between years, both in magnitude (7-fold) and timing (4 to 6 wk). Interannual differences in the under-ice abundance of nauplii were linked to variations in the abundance of female cyclopoid copepods, and appeared unrelated to the timing of the ice-algal or phytoplankton blooms. Interannual differences (2- to 4-fold) in the feeding success of fish larvae (percent feeding incidence at length and mean feeding ratio at length) were related to the availability of copepod nauplii. Consistent with the match/mismatch hypothesis, the fixity of the spawning season in relation to a variable cycle of prey abundance accounted for the observed variations in feeding success and apparent growth (length at date) of fish larvae. Yet, in this particular ecosystem, a match or mismatch between Arctic cod or sand lance larvae and their prey may depend more on the dynamics of cyclopoid copepods during the previous winter than on the timing of the spring algal blooms. (Résumé d'auteur

Impact of freshwater on a subarctic coastal ecosystem under seasonal sea ice (southeastern Hudson Bay, Canada) : 3. Feeding success of marine fish larvae

We monitored the feeding success (percent feeding incidence at length and mean feeding ratio at length) of Arctic cod (#Boreogadus saida) and sand lance (#Ammodytes sp. larvae in relation to prey density, light, temperature and potential predator density under the ice cover of southeastern Hudson Bay in the spring of 1988, 1989 and 1990. Both prey density and light limited larval fish feeding. The relationship between feeding success and actual food availability (nauplii density x irradiance) was adequately described by an Ivlev function which explained 64 and 76% of the variance in Arctic cod and sand lance feeding success respectively. By affecting both prey density and irradiance, the thickness of the Great Whale River plume (as defined by the depth of the 25 isohaline) was the main determinant of prey availability. Arctic cod and sand lance larvae stopped feeding when the depth of the 25 isohaline exceeded 9 m. Limitation of feeding success attributable to freshwater inputs occurred exclusively in 1988, the only time when the depth of the 25 isohaline exceeded the 9 m threshold. The close dependence of larval fish feeding success on the timing of the freshet and plume dynamics suggests a direct link between climate and survival of Arctic cod and sand lance larvae. The actual impact of climate fluctuations and/or hydro-electric developments on recruitment will depend on the fraction of the larval dispersal area of the two species that is affected by river plumes. (Résumé d'auteur

Dissociating between the N2pc and attentional shifting: an attentional blink study

The N2pc is routinely used as an electrophysiological index of attentional shifting. Its absence is thus taken as evidence that no shift of attention occurred. We provide evidence in contrast to this notion using a variant of the attentional blink (AB) paradigm. Two target letters, embedded in two streams of distractor letters and defined by their color, were separated by either 300 or 800 ms. The second target was preceded by a distractor frame of the same color (cue). As expected, identification of the second target was poorer at the short than at the long lag (the AB effect). The AB did not affect attentional capture by the cue, but suppressed and delayed the N2pc associated with it. This result suggests that the N2pc does not reflect attentional shifting. Instead, we conclude that the N2pc indexes the transient enhancement that occurs at the spatial focus of attention and promotes high-level processing such as identification. This conclusion calls for a reinterpretation of findings from the attentional capture literature that relied on the N2pc as an index of attentional shifting. Our results also inform contemporary models of the AB

Kinetics of 13C-DHA before and during fish-oil supplementation in healthy older individuals

Background: Docosahexaenoic acid (DHA) kinetics appear to change with intake, which is an effect that we studied in an older population by using uniformly carbon-13–labeled DHA (13C-DHA). Objective: We evaluated the influence of a fish-oil supplement over 5 mo on the kinetics of 13C-DHA in older persons. Design: Thirty-four healthy, cognitively normal participants (12 men, 22 women) aged between 52 and 90 y were recruited. Two identical kinetic studies were performed, each with the use of a single oral dose of 40 mg 13C-DHA. The first kinetic study was performed before participants started taking a 5-mo supplementation that provided 1.4 g DHA/d plus 1.8 g eicosapentaenoic acid (EPA)/d (baseline); the second study was performed during the final month of supplementation (supplement). In both kinetic studies, blood and breath samples were collected ≤8 h and weekly over 4 wk to analyze 13C enrichment. Results: The time × supplement interaction for 13C-DHA in the plasma was not significant, but there were separate time and supplement effects (P < 0.0001). The area under the curve for plasma 13C-DHA was 60% lower while subjects were taking the supplement than at baseline (P < 0.0001). The uniformly carbon-13–labeled EPA concentration was 2.6 times as high 1 d posttracer while patients were taking the supplement as it was at baseline. The mean (±SEM) plasma 13C-DHA half-life was 4.5 ± 0.4 d at baseline compared with 3.0 ± 0.2 d while taking the supplement (P < 0.0001). Compared with baseline, the mean whole-body half-life was 61% lower while subjects were taking the supplement. The loss of 13C-DHA through β-oxidation to carbon dioxide labeled with carbon-13 increased from 0.085% of dose/h at baseline to 0.208% of dose/h while subjects were taking the supplement. Conclusions: In older persons, a supplement of 3.2 g EPA + DHA/d increased β-oxidation of 13C-DHA and shortened the plasma 13C-DHA half-life. Therefore, when circulating concentrations of EPA and DHA are increased, more DHA is available for β-oxidation. This trial was registered at clinicaltrials.gov as NCT01577004

Life in the fast lane: Revisiting the fast growth—High survival paradigm during the early life stages of fishes

Early life survival is critical to successful replenishment of fish populations, and hypotheses developed under the Growth-Survival Paradigm (GSP) have guided investigations of controlling processes. The GSP postulates that recruitment depends on growth and mortality rates during early life stages, as well as their duration, after which the mortality declines substantially. The GSP predicts a shift in the frequency distribution of growth histories with age towards faster growth rates relative to the initial population because slow-growing individuals are subject to high mortality (via starvation and predation). However, mortality data compiled from 387 cases published in 153 studies (1971–2022) showed that the GSP was only supported in 56% of cases. Selection against slow growth occurred in two-thirds of field studies, leaving a non-negligible fraction of cases showing either an absence of or inverse growth-selective survival, suggesting the growth-survival relationship is more complex than currently considered within the GSP framework. Stochastic simulations allowed us to assess the influence of key intrinsic and extrinsic factors on the characteristics of surviving larvae and identify knowledge gaps on the drivers of variability in growth-selective survival. We suggest caution when interpreting patterns of growth selection because changes in variance and autocorrelation of individual growth rates among cohorts can invalidate fundamental GSP assumptions. We argue that breakthroughs in recruitment research require a comprehensive, population-specific characterization of the role of predation and intrinsic factors in driving variability in the distribution and autocorrelation of larval growth rates, and of the life stage corresponding to the endpoint of pre-recruited life. -- Keywords : critical period ; growth-mortality ; individual characteristics ; larval physiology ; predation ; recruitment endpoint