Maximizing opportunities and minimizing risks for children online: the role of digital skills in emerging strategies of parental mediation

As internet use becomes widespread at home, parents are trying to maximize their children’s online opportunities while also minimizing online risks. We surveyed parents of 6- to 14-year-olds in eight European countries (N=6,400). A factor analysis revealed two strategies. Enabling mediation is associated with increased online opportunities but also risks. This strategy incorporates safety efforts, responds to child agency and is employed when parent or child is relatively digitally skilled, so may not support harm. Restrictive mediation is associated with fewer online risks but at the cost of opportunities, reflecting policy advice that regards media use as primarily problematic. It is favoured when parent or child digital skills are lower, potentially keeping vulnerable children safe yet undermining their digital inclusion

Size-selective mortality of laboratory-reared Atlantic bluefin tuna larvae: evidence from microstructure analysis of otoliths during the piscivorous phase

Atlantic bluefin tuna (Thunnus thynnus) larvae show strong piscivorous feeding behavior at the very early larval stage and this enables them to grow at high rates. We conducted a laboratory experiment in which Atlantic bluefin tuna larvae were offered larval prey for the first time at different ages to simulate the early onset of piscivory at three treatments: yolk-sac larvae (YSL), delayed onset of piscivory (DYSL) and a solely planktivorous diet (Rotifers). The otolith microstructure was then used to compare the larval size distribution at the onset of the experiment with the estimated previous size-at-age of the survivors at the end of the experiment by back- calculation. Within a cohort, our results show size-selective mortality of the largest larvae independent of the differences in the timing of onset of piscivory and differences in growth patterns. The results also corroborate the rapid response of Atlantic bluefin tuna to piscivory in terms of growth reflected in the otolith increment widths. Being bigger did not infer a survival advantage and mortality rates did not decline with increasing larval size. Smaller size at a given age could under certain conditions and stages of development confer a survival advantage of individual members of a larval cohort when suitable small-sized prey is available.Versión del editor2,26

Experimental evidence on measures to protect consumers of online gambling services

Online gambling has grown rapidly in recent decades due to increased accessibility and availability. This article reports the results of a behavioral experiment conducted in a laboratory (N=522) and an online experiment administered in seven European countries (N=5997). The experiments examined the effectiveness of a range of mainstream and also innovative protective interventions for online gambling. The rationale of the interventions was to disrupt both individuals\u2019 mental processes and the affordances embedded in the human-machine system designed to maximize the time spent gambling and industry profits. Behavioral measures including stake size, speed of play and decision to stop playing or make further gambles were recorded. The results show that interventions addressing both individuals\u2019 mental processes and the human-machine interaction are effective in reducing the stake size and in slowing down the pace of gambling. All other interventions directed at the level of the individual have no effect on behavior. The results show that traditional \u2018nudges\u2019 are not sufficient and structural features such as the affordances embedded by design into the online gambling machines must be addressed in order to effectively protect consumers of online gambling

Recruitment processes in West Greenland waters: With special focus on Greenland halibut ( Reinhardtius hippoglossoides, W.)

The topic of this thesis is recruitment processes in Greenland Halibut (Reinhardtius hippoglossoides) in West Greenland waters. The focal point is Greenland halibut’s early life history from oocytes in the female ovary - to larvae living in the pelagic - to juveniles on the sea bottom. These early life stages are examined in four studies: Ovary and oocyte development in adult female Greenland halibut was studied in Davis Strait, Disko Bay and Baffin Bay. The objective of this study was to track and compare ovary development from winter to autumn from the inshore fjords of Disko Bay, Baffin Bay and the presumed spawning area in Davis Strait. Results showed that maturity was much more progressed in Davis Strait compared to the two other areas. Furthermore, almost all adult female fish in Davis Strait showed signs of maturation while only a fraction of the larger females (>80 cm length) were mature in Disko and Baffin Bay. A large proportion of the Greenland halibut in Disko and Baffin Bay did not appear to begin the maturation cycle until very late in their life history and/or were repeat spawners with a multiyear maturation cycle. Thus the observations from Disko and Baffin Bay support the theory that Greenland halibut can have a prolonged adolescent phase. The result from the study also strongly indicated that Davis Strait was the only major spawning area in West Greenland. Larvae distribution and growth of Greenland halibut and Sandeel (Ammodytes sp.) was studied across the West Greenland shelf. The objective of this study was to analyse spatial distribution and growth during three periods May, June and July and relate these to cross shelf variability in biological and hydrographical conditions in order to identify areas of special importance for larval growth and survival. Results showed that in May small Greenland halibut larvae, some still with small remains of the yolk-sac, were primarily distributed offshore in Davis Strait. Their distribution coincided with their prey resources while some vagrant larvae were dispersed to areas with less food, which likely resulted in higher mortality rates. Consequently, the emergence of first feeding Greenland halibut larvae matched their prey in time and space which is believed to be crucial for survival success at this critical stage in their development. From June to July Greenland halibut larvae gradually shifted their distribution from the bank slopes to the deeper parts of the slope near the shelf break. The lack of clear and well-defined frontal zones made it difficult to evaluate the hydrographical processes that had significance for larval distribution patterns. Greenland halibut larvae did, however, show preference for stratified water masses. Growth analysis of Greenland halibut larvae was only possible for July and showed higher growth was observed at stations positioned in frontal zones suggesting that growth was significantly influenced by the level of frontal activity. The study also included distribution and growth comparisons with sandeel. The larvae of sandeel was considerably more associated to the banks and bank slopes than Greenland halibut larvae in all three months (May-July), however in June, the two species coexisted considerably in the same areas of the cross shelf. The growth analysis showed that sandeel and Greenland halibut larvae showed opposite responses to some hydrographical conditions and prey abundance. Sandeel growth decreased with increasing frontal activity and increased with increasing prey biomass. The later observation suggests that sandeel growth was limited by food availability. Cross shelf variation in growth in Greenland halibut larvae showed that enhanced growth was observed at some stations where they were most abundant. Overall, however, there was no clear association between larval growth and larval abundances for neither Greenland halibut nor sandeel larvae. The observed patchiness in larval distribution could therefore also be a result of retention mechanisms and / or predation mortality. The feeding ecology of pelagic Greenland halibut and sandeel larvae was studied across the West Greenland shelf. The aim of this study was to analyse and compare feeding ecology of coexisting Greenland halibut and sandeel larvae. Diet analysis and prey preference considerations were used to assess spatial variation in prey availability and feeding. The hypothesis was that feeding conditions are optimal in central areas of fish larval distribution, and that timing of larval emergence is synchronised with prey availability. The study showed that for both species copepods were the main prey items and that absolute size of preferred prey increased during larval ontogeny. However, preferred copepod size in relation to larval length differed markedly between Greenland halibut and sandeel. In Greenland halibut the relative size of the prey declined during growth of the larvae while for sandeel the relative size of prey remained constant at a level of 2.7% of larval length. This led to a reduction in prey niche overlap between the two species as Greenland halibut and sandeel larvae increased in size. The available prey copepod biomass differed distinctly across the shelf area. In May, Greenland halibut prey density was highest in the off-shelf area in Davis Strait. In June and July, the prey rich areas for both species were mainly located on the slopes of the banks and at the shelf break area. In these areas gut fullness was higher than in neighbouring areas, suggesting that the food resource could be scarce. The feeding ecology of Greenland halibut and sandeel could explain why larval abundance indices for the two species historically have shown opposite responses to annual environmental conditions and total zooplankton occurrence. Drift and growth of Greenland halibut from eggs throughout the pelagic larval stage was tracked from sub-population spawning areas in Davis Strait, Baffin Bay, East Greenland and West Iceland by combing drift- and individual-based egg and larval temperature dependent growth models. Model results were compared to survey results and the settling process was followed at Store Hellefisk Bank in West Greenland. The aim of this study was to evaluate spatio-temporal distribution of small first feeding and large settling larvae, and the between metapopulations. Results showed that egg and larvae can drift for long distances and that the exchange of individuals among the geographically-separated sub-populations could be important for metapopulation dynamics. Larvae from the spawning area in Davis Strait were primarily (>60%) transported to Canadian waters. Only larvae from the eastern part of the spawning area in Davis Strait remained in West Greenland. All larvae from the East Greenland spawning area were transported south of Cape Farewell and to West Greenlandic (82%) or Canadian (18%) waters. From the West Icelandic spawning areas larvae either, depending on the spatial location on the emergence first feeding larvae, drifted to East Greenland (45-53%), West Greenland (19−34%) or Northern Iceland (21−28%) or remained (>98%) in Iceland waters. In Baffin Bay low water temperatures resulted in very slow development rates and eggs probably never hatched. The surveys showed that in West Greenland larvae settling started in August and continued in September but the settling peak period could not be determined. The study in the two areas of Hellefisk Bank showed that juveniles of age 1 and 2+ almost exclusively occurred in one area while the 0-group was equally distributed in both areas. In the area of high juvenile age 1 and 2+ abundance the condition and stomach fullness was significantly higher than in the low abundance areas. Nursery ground processes within the first year clearly restricted the successful nursery area to a more specific bottom habitat types. The concentrating of juveniles into specific nursery areas implies that juvenile densities may approach the carrying capacity of their habitats in years when settlement is high, which would dampen the annual variability in year class strength of Greenland halibut

Otolith microstructure analyses in cultured Atlantic bluefin tuna larvae as a tool to provide accurate estimates of size selective growth and mortality

Being bigger is not always the best option to survive and mortality rates do not always decline with increasing larval size. Smaller size at a given age could under certain condition and at certain stages of development confer a survival advantage on individual members of a larval cohort. Reference

The effect of nutritional condition on the growth to post-flexion of bluefin tuna larvae under cultured condition

These findings indicate that protein synthesis increase in proportion to the flexion stage which is strongly associated with the development of the digestive system and therefore the increase in the nutritional condition. Ontogenetic differences in the RNA and DNA content will be discussed in more detail. Also, the pattern obtained for Atlantic bluefin tuna larva will be compared with those of other species

The effect of different short pulse feeding regimes on growth and survival of Atlantic bonito larvae Sarda sarda

One of the most easily manipulated variables in fish larval culture is the photoperiod. Long light photoperiod regimes are commonly used to enhance growth in commercial species. However, for species with a piscivorous larval period, as the Atlantic bonito (Sarda sarda), long time exposure to light could lead to a lower survival through aggressive behavior and cannibalism. One alternative could be modifications of the light and dark cycles during the photophase. These modifications can result in short pulse feeding regimes since bonito larvae fill up their stomach completely during light hours but do not feed in darkness. Little is known about how such intermittent feeding regimes affect growth and survival in fish. In this study, we tried different alternating and continuous light regimes during the culture of bonito larvae to identify the best regime that maximizes growth and survival.Fertilized eggs of Atlantic bonito were obtained from different spontaneous spawning events by a captive broodstock at the Spanish Institute of Oceanography (IEO) in Mazarrón. Bonito larvae 8 days post hatch (dph) were reared in 150 l tanks equipped with a lid that was used to cover and uncover the tanks to manipulate the hours of light and therefore pulse feeding regimes. Bonito larvae were always fed with yolk sac seabream larvae ad libitum. A total of three experiments were conducted. In all, a continuous dark period of 7.5hours was maintained from 24:00 to 7:30. All light regimes had a total of 9 light hours except for one that had 15hours of light (15L:9D). In the first experiment, light regimes provided alternating light and dark conditions of either 1.5, 3 or 4.5hours from 7:30 to 24:00). In the second experiment, the 3hours alternating light regime was compared to two continuous regimes of 15hours of light (15L:9D) and 9hours of light (9L:15D). These two experiments were conducted at the same temperature, 24.7±0.4°C. The third experiment was identical to the second experiment but at colder temperatures, 21.4±0.45°C. All regimes had 3 replicates. 10 larvae were sub-sampled 3 days after the experiments began and were ended after 6 days when all larvae were sampled. Due to slower growth, the third experiment ended after 9 days. The larvae were measured in standard length and individual dry weights were calculated. Larvae were counted in the tanks every 3 days to estimate survivorship. Final larval sizes in the alternating light regimes were larger in the 3hours than those obtained in the 1.5 and 4.5hours (first experiment, Fig. 1a, Tukey test p0.01). The 3hours alternating light regime yielded the largest larval sizes at the end of the experiment compared to the other alternating regimes. Final sizes at the 3hours regime were larger than those obtained under the 9L:15D continuous light regime at both temperatures. The time to satiation and the elapsed time to evacuate food totally from the gut in a similar species is about 3-4hours (Young and Davis, 1990). Our results suggest that a better strategy for bonito larvae growth is to fill their stomach more than once per day followed by a resting period when food is being digested. However, no effect was observed in terms of survival, possibly due to high abundance of larval prey. Changes in the light regime that result in pulse feeding can be a good strategy to increase growth in larval cultures when fitting well the evacuation and satiation rates

Metabolic rates in Atlantic bluefin tuna larvae: first data and methodological challenges

Atlantic bluefin tuna is an emblematic scombrid species, but many physiological aspects during the larval stages are still unknown. The mechanisms of how fish larvae balance growth and activity are of great interest since metabolic costs are very high compared to juveniles and adults. However, there is a lack of information about metabolic costs in scombrid larvae. This lack of data is probably related to the challenges associated to larval handling before and during respirometry trials. In this study for the first time, we: i) estimate the relationship between routine metabolic rate and the larval dry weight (mass scaling exponent) at 26°C, ii) measure metabolism under light and darkness and iii) explore the influence of nutritional status (RNA:DNA ratio) on the inter-individual variability in metabolic rates. The relationship between metabolism and size (ranging from 0.6 to 23 mg) was near isometric (slope, b=0.99), in contrast to the allometric relationship observed in most species (b=0.87). Our results show no significant differences in oxygen consumption under light and darkness. A possible regulation (decrease) of their swimming activity with the consequent decrease in the oxygen consumption in light situations is discussed. Nutritional condition did not explain the inter-individual differences in oxygen consumption. This study first reports metabolic rates of Atlantic bluefin tuna larvae and discusses the challenges of performing bioenergetics studies with early life stages of Scombrids