Timing of the maturation transition in haddock Melanogrammus aeglefinus

Tracking oocyte development and the timing of skipped spawning for north-east Arctic haddock (Melanogrammus aeglefinus)

The present study tracked oocyte development over 9 months and noted incidences of 'skipping', i.e., adults terminating their upcoming reproductive cycle, in field-caught north-east Arctic (NEA) haddock (Melanogrammus aeglefinus), currently the largest stock of this species. Applications of advanced image and histological techniques revealed the presence of cortical alveoli oocytes (CAO), which prevailed as the most advanced oocyte phase for 4-5 months. This new finding of an extended and early appearance of CAOs in this gadoid was supported by that vitellogenesis first started to appear 3 months later. The subsequent oocyte growth trajectories indicated that larger individuals [total length (TL) = 70 cm] typically spawn in the order of 3 weeks earlier than the smaller ones (TL = 40 cm). The spawning season appeared stretched over about 3 months. The majority of skipping females arrested oocyte growth at the CAO phase followed by atretic reabsorption. Compared to those individuals maturing for the spawning season, 'skippers' generally exhibited lower body condition, characterized also by relatively lower liver sizes at the time of the main spawning season. This study demonstrated well-developed skipping dynamics, but also that the CAO period, i.e., when skipping takes place, may be exceedingly long in this commercially valuable gadoid and that its reproductive cycle in many ways deviates from that of the data-rich, sympatric NEA cod (Gadus morhua).

Effects of ambient oxygen and size-selective mortality on growth and maturation in guppies

Growth, onset of maturity and investment in reproduction are key traits for understanding variation in life-history strategies. Many environmental factors affect variation in these traits, but for fish, hypoxia and size-dependent mortality have become increasingly important because of human activities, such as increased nutrient enrichment (eutrophication), climate warming and selective fishing. Here, we study experimentally the effect of oxygen availability on maturation and growth in guppies (Poecilia reticulata) from two different selected lines, one subjected to positive and the other negative size-dependent fishing. This is the first study to assess the effects of both reduced ambient oxygen and size-dependent mortality in fish. We show that reduced ambient oxygen led to stunting, early maturation and high reproductive investment. Likewise, lineages that had been exposed to high mortality of larger-sized individuals displayed earlier maturation at smaller size, greater investment in reproduction and faster growth. These life-history changes were particularly evident for males. The widely reported trends towards earlier maturation in wild fish populations are often interpreted as resulting from size-selective fishing. Our results highlight that reduced ambient oxygen, which has received little experimental investigation to date, can lead to similar phenotypic changes. Thus, changes in ambient oxygen levels can be a confounding factor that occurs in parallel with fishing, complicating the causal interpretation of changes in life-history traits. We believe that better disentangling of the effects of these two extrinsic factors, which increasingly affect many freshwater and marine ecosystems, is important for making more informed management decisions.

How to measure maturation: a comparison of probabilistic methods used to test for genotypic variation and plasticity in the decision to mature

Maturation is a developmental trait that plays a key role in shaping organisms' life-history. However, progress in understanding how maturation phenotypes evolve has been held back by confusion over how best to model maturation decisions and a lack of studies comparing genotypic variation in maturation. Here, we fitted probabilistic maturation reaction norms (PMRNs) to data collected from five clones of Daphnia magna and five of Daphnia pulex collected from within and between different populations. We directly compared the utility of modeling approaches that assume maturation to be a process with an instantaneous rate with those that do not by fitting maturation rate and logistic regression models, and emphasize similarities and differences between them. Our results demonstrate that in Daphnia, PMRNs using a logistic regression approach were simpler to use and provided a better fit to the data. The decision to mature was plastic across a range of growth trajectories and dependent upon both body size and age. However, the age effect was stronger in D. magna than D. pulex and varied considerably between clones. Our results support the idea that maturation thresholds can evolve but also suggest that the notion of a threshold based on a single fixed state is an oversimplification that underestimates the adaptability of these important traits.

Implications of a warming North Sea for the growth of haddock Melanogrammus aeglefinus

The present study aimed firstly, to test for a temperature effect on North Sea haddock Melanogrammus aeglefinus growth and secondly, to develop a model that could be used to assess total length (L(T)) and mass (M)-at-age response to different temperature scenarios. The von Bertalanffy growth model was fitted on a cohort-by-cohort basis from 1970 to 2006. The asymptotic L(T) (L(∞)) was negatively correlated with temperature while the rate at which L(∞) is reached (K) was positively correlated with temperature. K was negatively correlated with density, whereas no effect on L(∞) was observed. These effects were incorporated into a von Bertalanffy model which was extended to include temperature and density as explanatory variables. Only the temperature variable was significant. Fitting the extended von Bertalanffy model revealed that L(∞) decreased while K increased with increasing temperature, resulting in up to a 40% loss of individual yield at older ages. The dramatic decline observed in the mean age at which 50% of the population becomes mature suggests that higher temperatures resulted in larger young M. aeglefinus that matured earlier and therefore reached a smaller maximum size. In a global warming context, the loss of individual yield observed at old ages is likely to reduce the fisheries yield for M. aeglefinus in the North Sea.