Variation In Size Structure Research Articles

Spatiotemporal variation in the size structure of the chub mackerel, Scomber japonicus, over half a century in the East Asian Marginal Seas

The size and age structure of a population are crucial aspects for understanding population dynamics. Many marine fish species have experienced long-term reductions in size and age owing to overfishing and/or global warming; however, these aspects remain unexplored. The present study elucidated the spatiotemporal variations in chub mackerel size and age structure based on the operational reports of large- and medium-sized Japanese purse seine fisheries for nearly half a century. Our results revealed size- and age-dependent segregation at a fine spatial scale, which may indicate ontogenetic niche shifts in response to thermal and feeding conditions or intercohort competition. On a long-term temporal scale, a significant reduction in size and age was observed in the East China Sea, whereas a contrasting increase in size and age was observed in the northern Sea of Japan. This may indicate that large fish have greater sensitivity to global warming and shift their distribution more rapidly northward than do small fish. In addition, the reduction in size and age during the spawning season in the East China Sea may result in a bias towards younger spawners, potentially rendering population dynamics more unstable. This study highlights the importance of investigating the spatiotemporal variation in the size structure of exploited fish stocks.

Seasonal variation in size structure and production of autotrophic plankton community in eutrophied, low-light environment: A focus on Mesodinium rubrum

Temporal variations in the primary production of the size-fractionated autotrophic plankton community were studied in coastal-estuarine waters of the eutrophic Gulf of Riga, Baltic Sea. The community was net-autotrophic during spring and summer and net-heterotrophic during autumn. The results of the present study clearly demonstrate strong covariation between net primary production (NPP) and <56 µm fractionated community biomass, particularly small-sized (16–33 µm) Mesodinium rubrum, implying that the majority of NPP stems from the lower end of the size spectrum. A pronounced size distribution shift was observed within the M. rubrum population. Large-sized (length ≥34 µm) M. rubrum was the most abundant in the first half of the productive season (until week 24), whereas small-sized M. rubrum dominated during the stratified period.

Recovery of the salt marsh periwinkle (Littoraria irrorata) 9 years after the Deepwater Horizon oil spill: Size matters

Prior studies indicated salt marsh periwinkles (Littoraria irrorata) were strongly impacted in heavily oiled marshes for at least 5 years following the Deepwater Horizon oil spill. Here, we detail longer-term effects and recovery over nine years. Our analysis found that neither density nor population size structure recovered at heavily oiled sites where snails were smaller and variability in size structure and density was increased. Total aboveground live plant biomass and stem density remained lower over time in heavily oiled marshes, and we speculate that the resulting more open canopy stimulated benthic microalgal production contributing to high spring periwinkle densities or that the lower stem density reduced the ability of subadults and small adults to escape predation. Our data indicate that periwinkle population recovery may take one to two decades after the oil spill at moderately oiled and heavily oiled sites, respectively.

Changing Occurrences of Fall Blooms Associated With Variations in Phytoplankton Size Structure in the Pacific Arctic

Phytoplankton blooms in the Pacific Arctic have been characterized as a huge single bloom in spring. However, several studies have reported recent increases in the occurrence of fall blooms during the period after dissipation of the spring bloom. Here, we explored spatiotemporal variations in the occurrence of fall blooms in the region, using satellite remote-sensing data for 2003–2017. Seasonal time-series variation in satellite-derived chlorophyll-a concentrations (chla) was modeled using a Gaussian function to distinguish whether a peak in chla was evident in fall; an occurrence of a fall bloom was recognized if the model detected the presence of a chla peak in fall. In addition, phytoplankton size structure was estimated from the satellite data to examine the influence of fall blooms on seasonal variations in the size structure. The results indicate that fall blooms occurred in a wide area of the Pacific Arctic, and larger phytoplankton were predominant during fall blooms relative to the phytoplankton present before/after the bloom or in the absence of a fall bloom. Examining interannual variation in occurrences of fall blooms revealed clear increasing and decreasing trends in the southern Chukchi Sea and the St. Lawrence Island polynya region, respectively, possibly associated with temporal variations in atmospheric forcing as well as in the water-column structure. Because the cell sizes of phytoplankton largely determine their sinking rate, temporal trends in the occurrence of fall blooms modulating seasonal variations in phytoplankton size structure could significantly influence marine ecosystems. These results suggest that spatiotemporal monitoring of phytoplankton communities not only in spring but also after the period or their dissipation would improve our understanding about processes causing variations within marine ecosystems, as might occur in the Pacific Arctic.

Spatial patterns of zooplankton size structure in relation to environmental factors in Jiaozhou Bay, South Yellow Sea.

The spatial variation in size structure of mesozooplankton and its relationship with environmental factors were studied from January 2006 to December 2010 in Jiaozhou Bay. The spatial body-size patterns of zooplankton differed significantly from the inner to the outer bay, and have a significant correlation with environmental variables, especially nutrients, salinity and water depth. The abundance of total zooplankton and small size ranks presented a decreasing gradient from the inner to the outer part, and was positive correlated with nutrients and negative with salinity, while the abundance of large size ranks exhibited an opposite gradient, and was positive correlated with water depth and negative with nutrients. The size spectra slope and size diversity were generally low in the inner part and closely related to body size ranks. Our results suggest that size structure patterns of zooplankton showed a clear spatial pattern and significantly associated with environmental factors in marine ecosystems.

Fitting methods and seasonality effects on the assessment of pelagic fish communities in Daya Bay, China

Abstract The general relationship indicating that biomass decreases with individual body size is referred to as size spectra. This is consistent with the power law that characterizes size frequency distributions. While many previous studies have used the estimated exponent in size spectra as a metric to assess external perturbations, limited empirical studies have focused on the fitting methods. Here, we compared the effects caused by fitting methods and distribution models based on one-year pelagic fish data from Daya Bay, China. Our empirical results showed that maximum likelihood estimation (MLE) is more suitable than traditional normalized biomass spectra (NBS), and that a power law is not always the best model when using MLE. Moreover, we found significant size structure variation in different seasons. Principal component analysis (PCA) and linear mixed effects model (LMM) results showed that temperature was the major factor in seasonal environmental variation, and fish migration might be the essential response strategy causing size structure changes. The estimated exponents and nonlinear structure indicated that the Daya Bay is under intensive human impact. Our results suggest that MLE methods are recommended in future size-based studies, and that environmental variation and migration patterns are crucial in understanding seasonal community structure changes.

Spatial Variability in Size Structure, Growth, and Recruitment of Spotted Seatrout among Six Florida Estuaries

AbstractSpotted Seatrout Cynoscion nebulosus in Florida, USA, are managed under four geographical regions, including Apalachicola Bay and Cedar Key (northwest region), Tampa Bay and Charlotte Harbor (southwest region), northeast Florida (northeast region), and the northern Indian River Lagoon (southeast region). Two of these management regions are composed of more than one major estuary system (northwest and southwest regions). However, previous life history research suggests that the management regions do not accurately reflect the unique biological populations of Spotted Seatrout in Florida. Our objective was to determine (1) whether there is significant spatial variation in size structure, the age–length relationship, and recruitment patterns among populations of Spotted Seatrout in Florida; and (2) whether there is agreement in these life history parameters among estuary populations comprising individual management regions. We used long‐term fishery‐independent and fishery‐dependent data sets on recruitment, age, and size structure among six estuaries and the four management regions. Young‐of‐the‐year recruitment patterns and the length‐at‐age relationship over all ages differed significantly among estuarine populations and within management regions. Additionally, all estuaries differed significantly in length distributions of Spotted Seatrout caught by the recreational fishery. Our results highlight the need to understand the relative contributions of estuary‐specific data to regional assessment models if the four‐region boundary scheme for Florida is continued. Moreover, as variation in growth and recruitment among estuaries likely reflects estuary‐specific environmental conditions, we suggest that environmental variables be considered in future assessments and management of this recreationally valuable species.

Variability of size structure and species composition in Caribbean octocoral communities under contrasting environmental conditions

Octocorals have increased in abundance on a number of Caribbean reefs, but this trend has largely been reported with functional group or genus resolution. A species-level analysis of octocoral communities in St. John, US Virgin Islands was conducted to better understand how this taxon will respond to changing conditions based on their synecology at two sites that are 1.5 km apart and differ in physical conditions. East Cabritte is characterized by moderate wave energy, low sedimentation, and clear water, while contrasting conditions characterize Europa Bay. Surveys conducted in 2014 and 2015 showed that the abundance and size of adult octocorals differed between sites, with taller and denser communities at East Cabritte than Europa Bay (mean height of 32 versus 20 cm; 18 versus 8 colonies m−2). Octocoral diversity and evenness were similar between sites, although multivariate octocoral community structure differed between sites regardless of whether octocorals were resolved to genera or species. Genus-resolution masked differences between sites for speciose genera like Eunicea. The broad overlap in species representation at both sites suggests that diversity is less responsive than community structure to differing environmental conditions, perhaps because the ecological niches of these species are broad. With 35 octocoral species, and diversity and abundances comparable to those studied > 40 years ago on shallow Caribbean reefs, the dense octocoral communities appearing on some present-day reefs reflect expanded benthic occupancy by a well-known (rather than a novel) community type.

VARIACIÓN INTERANUAL DE LA ESTRUCTURA DE TALLAS Y ASPECTOS REPRODUCTIVOS DEL CAMARÓN PINK (FARFANTEPENAEUS BREVIROSTRIS) EN EL PACÍFICO COLOMBIANO

The fishery of the deep water shrimp is a resource of great economic and social importance for the Colombian Pacific Coast. Farfantepenaeus brevirostris (pink shrimp) is the target resource supporting this fishery; however, reproductive information and the fishing effect on size structure with management implications, has not been documented in recent years. The aim of this study was to analyze the variation in size structure and reproductive aspects of F. brevirostris, therefore 45 hauls were sampled in three onboard fishery monitoring of the industrial trawl fleet per year, between 2009 and 2015. In each haul in situ morphometric characteristics and sexual maturity of 20262 individuals were recorded. 56.2% of individuals collected were females, however the ratio female: male of 1:0.8 was not significant. The size range caught by the fishing fleet did differ between sexes and years, showing that females were larger (50 mm and 200 mm in total length) than males (75 mm to 170 mm). In this regards, females showed the highest mean catch size in 2012 (128.04 mm) and the lowest in 2011 (114.9 mm), while males were larger in 2011 (166.6 mm) and smaller in 2015 (106.3 mm). The maturity function had a different pattern between sexes and constant variability in maturity curves among years, which meant estimating the average size at maturity (TMM) for F. brevirostris based on an average curve of seven years (TMM females = 130.6 mm and TMM males = 83.2 mm). The size structure obtained and maturity curves indicated that females are more susceptible to fishing (between 66% and 94% of immature individuals are harvested). However, catches showed that only 1% to 32% of males are immature, which enables a success reproductive strategy of the population to offset the fishing effect. Better control of gear selectivity and implementation of spatial-temporal fishing closures are suggested for the management and conservation of this resource.

Non-stationary recruitment dynamics of rainbow smelt: The influence of environmental variables and variation in size structure and length-at-maturation

Fish stock-recruitment dynamics may be difficult to elucidate because of nonstationary relationships resulting from shifting environmental conditions and fluctuations in important vital rates such as individual growth or maturation. The Great Lakes have experienced environmental stressors that may have changed population demographics and stock-recruitment relationships while causing the declines of several prey fish species, including rainbow smelt (Osmerus mordax). We investigated changes in the size and maturation of rainbow smelt in Lake Michigan and Lake Huron and recruitment dynamics of the Lake Michigan stock over the past four decades. Mean lengths and length-at-maturation of rainbow smelt generally declined over time in both lakes. To evaluate recruitment, we used both a Ricker model and a Kalman filter-random walk (KF-RW) model which incorporated nonstationarity in stock productivity by allowing the productivity term to vary over time. The KF-RW model explained nearly four times more variation in recruitment than the Ricker model, indicating the productivity of the Lake Michigan stock has increased. By accounting for this nonstationarity, we were able identify significant variations in stock productivity, evaluate its importance to rainbow smelt recruitment, and speculate on potential environmental causes for the shift. Our results suggest that investigating mechanisms driving nonstationary shifts in stock-recruit relationships can provide valuable insights into temporal variation in fish population dynamics.

Population traits of invasive bleak Alburnus alburnus between different habitats in Iberian fresh waters

The bleak Alburnus alburnus (L.) is a cyprinid native to most of Europe, mainly inhabiting lentic environments. This fish species is a successful invader in the Iberian Peninsula, where it was first introduced to reservoirs as forage fish during the 1990s. Bleaks threaten the highly endemic Iberian fish fauna by means of trophic competition and hybridization. Yet, little is known about the environmental biology of bleaks in the Iberian Peninsula, particularly far from impounded waters. Thus, the aim of this work was to compare seasonal and gender variation of size structure, body condition and reproductive investment of bleaks between different habitats. Only sexually mature bleaks were seasonally collected and examined from the River Gévora and the Sierra Brava Reservoir (southwestern Spain) to assess more in-depth the adaptive capacity at the population level and the subsequent invasiveness. Bleak was an abundant species in the fish assemblages of both habitat types (i.e. river and reservoir). The proportion of smaller mature bleaks was lower in the river than the reservoir during spring and the opposite pattern was observed during winter. Both male and females were larger in the river during the breeding season in the study areas (i.e. spring), as well as with higher body condition and reproductive investment. These findings suggest that bleaks enhance their reproduction rate in the river to compensate for higher mortality in this habitat, where environmental conditions may be harsher due to the winter floods and summer droughts typical of Mediterranean water courses. Overall results highlight the high degree of plasticity in population traits of the bleak in the Iberian Peninsula, which will surely aid its ability to adapt to a wide variety of Mediterranean ecosystems, including lentic and lotic environments. Consequently, this invasive fish may pose a serious risk for the highly valuable fauna of Mediterranean Europe.

Reproductive biology of an amphidromous goby, Sicyopterus lagocephalus, in La Réunion Island

In the Indo-Pacific region, rivers are inhabited by amphidromous gobies. They are often subjected to a heavy fishing pressure, highlighting the urgent need to acquire knowledge on their biology for management purposes. This study investigated the reproductive strategy of Sicyopterus lagocephalus, a widespread amphidromous goby, in two rivers of La Reunion Island. Histological observations of ovarian samples and oocyte-size frequency distributions revealed that females had a group-synchronous ovarian follicle development (i.e., two cohorts of oocytes were distinguished simultaneously in ovaries). Females laid an entire clutch in a unique event (from 14,304 to 232,475 eggs) and then another batch of oocytes was recruited, showing that the annual fecundity is indeterminate. Ovarian growth was isometric at all oocyte development stages showing that the gonadosomatic index (GSI) is a good proxy of reproductive condition. The main reproductive season of S. lagocephalus spanned from early February to May with a narrower range upstream and a wider one downstream. Reproduction activity is mainly restricted by water temperatures in upstream areas, whereas it is likely influenced by female body condition and competition in downstream areas. The variability of reproductive traits leads to size structure variation in stocks of spawning females throughout the year.

Analysis of Variations in Phytoplankton Community Size-Structure along a Coastal Trophic Gradient

Kocum, E. and Sutcu, A., 2014. Analysis of variations in phytoplankton community size-structure along a coastal trophic gradient. The phytoplankton community size structure is a significant factor in the ecological and biogeochemical functioning of marine ecosystems. Various environmental factors, including nutrients, can cause variations in the size structure of phytoplankton community, ultimately determining the potential for downward carbon transport and productivity in marine ecosystems. In the present study, microscale spatial variation in phytoplankton size structure was investigated at two coastal stations having different nutrient environments. The variation in phytoplankton size structure was characterized by a significantly (p < 0.05) higher contribution of larger cells and diminishing abundance of picoplankton under increasing nutrient concentrations observed at both sites, although at different threshold unfractionated phytoplankton chlorophyll a biomass levels. The average percent contributions of picoplankton to total biomass were 30.02 and 19.07% at less enriched site I and at enriched site II, respectively. Thus, we concluded that the variation in phytoplankton size structure was led by the factors that regulated the contribution of large phytoplankton cells; that is, the dominance of picoplankton was only possible when nutrient concentrations precluded accumulation of the larger size classes in the phytoplankton community. Bu çalışmada Çanakkale Boğazı kıyılarında trofik bakımdan farklılık gösteren iki bölgede fitoplankton komünitesi büyüklük yapısındaki değişim incelenmiştir. Nutrient bakımından zengin bölgede ve toplam fitoplankton biyokütlesi arttığında ise her iki bölgede fitoplankton komünitesindeki büyük hücreli üyelerin bolluğunda artma ve pikoplankton bolluğunda da düşüş gözlemlenmiştir. Çalışma sonucunda fitoplankton komünitesi büyüklük yapısındaki varyasyonun büyük hücreli üyelerin artışını engelleyen koşullarca düzenlendiği yani, pikoplankton bolluğundaki artışın ancak büyük hücreli fitoplankton artışını sınırlayan nutrient yoğunluklarında meydana geldiği ileri sürülmüştür.

Characterizing the biological traits and life history of Acanthopagrus (Sparidae) hybrid complexes: implications for conservation and management

This study examined the spatio-temporal reproductive patterns, population structure, maturity and growth of Acanthopagrus hybrid complexes, which comprise mainly black bream Acanthopagrus butcheri and the hybrids they form with yellowfin bream Acanthopagrus australis, in two Australian estuaries (Coila and Brou Lakes, NSW). There were no differences between pure A. butcheri and hybrids in terms of their population structure, growth and maturity, suggesting that these two breeds have similar life histories and may therefore be managed as single, naturally cohesive units. Sexual variation in size structure was only observed for the complex in Coila Lake. Although there was significant variation in age structure between estuaries, both complexes exhibited dominance of only a few year classes indicating episodic recruitment and high fishing mortality. Acanthopagrus eggs occurred in higher numbers within upstream creek habitats compared to lake habitats, thereby highlighting the need to protect these spawning habitats. Although maturity in the complexes was not influenced by gender or estuary, differences in growth were detected between the genders and estuaries.

Seasonal shifts in predator body size diversity and trophic interactions in size‐structured predator–prey systems

1. Theory suggests that the relationship between predator diversity and prey suppression should depend on variation in predator traits such as body size, which strongly influences the type and strength of species interactions. Prey species often face a range of different sized predators, and the composition of body sizes of predators can vary between communities and within communities across seasons. 2. Here, I test how variation in size structure of predator communities influences prey survival using seasonal changes in the size structure of a cannibalistic population as a model system. Laboratory and field experiments showed that although the per-capita consumption rates increased at higher predator-prey size ratios, mortality rates did not consistently increase with average size of cannibalistic predators. Instead, prey mortality peaked at the highest level of predator body size diversity. 3. Furthermore, observed prey mortality was significantly higher than predictions from the null model that assumed no indirect interactions between predator size classes, indicating that different sized predators were not substitutable but had more than additive effects. Higher predator body size diversity therefore increased prey mortality, despite the increased potential for behavioural interference and predation among predators demonstrated in additional laboratory experiments. 4. Thus, seasonal changes in the distribution of predator body sizes altered the strength of prey suppression not only through changes in mean predator size but also through changes in the size distribution of predators. In general, this indicates that variation (i.e. diversity) within a single trait, body size, can influence the strength of trophic interactions and emphasizes the importance of seasonal shifts in size structure of natural food webs for community dynamics.

Effects of size structure and habitat complexity on predator-prey interactions

1. A predator's ability to suppress its prey depends on the level of interference among predators. While interference typically decreases with increasing habitat complexity, it often increases with increasing size differences among individuals. However, little is known about how variation in intrinsic factors such as population size structure alters predator–prey interactions and how this intrinsic variation interacts with extrinsic variation. 2. By experimentally varying the level of vegetation cover and the size structure of the predatory damselfly Ischnura posita Hagen, we examined the individual and interactive effects of variation in habitat complexity and predator size structure on prey mortality. 3. Copepod prey survival linearly increased as the I. posita size ratio decreased and differed by up to 31% among different predator size structures. Size classes had an additive effect on prey survival, most likely because intraspecific aggression appeared size-independent and size classes differed in microhabitat preference: large I. posita spent 14% more time foraging on the floor than small larvae and spent more time in the vegetation with increasing habitat complexity. Despite this difference in microhabitat use among size classes, habitat structure did not influence predation rates or interference among size classes. 4. In general, results suggest that seasonal and spatial variation in the size structure of populations could drive some of the discrepancies in predator-mediated prey suppression observed in nature, and this variation could exceed the effects of variation in habitat structure.

The consequences of size dependent foraging for food web topology

A general question in biology is how processes at one scale, for example that of individual organisms, influence patterns at larger scales, for example communities of interacting individuals. Here we ask how changing the size‐dependence of the foraging behaviour of individuals can influence the structure of food webs. We assembled communities using a model in which species interactions are determined by allometric foraging rules of (1) handling time and (2) attack rates, and also (3) the distribution of body sizes. We systematically varied these three factors and examined their effects on three community level, food web allometries: the generality ‐ mass correlation, the vulnerability ‐ mass correlation and the trophic height ‐ mass correlation. The results demonstrate how allometries of individual foraging behaviour (handling time and attack rates) are linked across scales of organisation: different community level allometries are influenced by different individual level allometries. For example, generality allometries in the community are most affected by the individual allometric relationships of the attack rate, whereas trophic level allometries in the community are more strongly influenced by variation in individual handling time allometries. Importantly, we also find that the shape of the body size distribution from which species are drawn has a substantial influence on how these links between scales operate. This study suggests that understanding the variation of size structure among ecological networks requires knowledge about the causes of variation in individual foraging behaviour and determinants of the regional body size distribution.

Latitude and coastline shape correlate with age-structure of Chlorostoma (Tegula) funebralis populations

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 424:133-143 (2011) - DOI: https://doi.org/10.3354/meps08993 Latitude and coastline shape correlate with age-structure of Chlorostoma (Tegula) funebralis populations Erin E. Cooper*, Alan L. Shanks University of Oregon, Oregon Institute of Marine Biology, PO Box 5389, Charleston, Oregon 97420, USA *Email: ecooper@uoregon.edu ABSTRACT: Between 2005 and 2008, size-frequency distributions from 22 populations of the intertidal snail Chlorostoma (Tegula) funebralis were studied to determine the effects of latitudinal region and coastal topography. Upwelling, which may affect larval recruitment, varies along a latitudinal gradient, while coastal topography may mitigate the effects of latitude. Sites were categorized by latitude: Northern (Oregon, 15 sites), and Southern (California, 5 sites; Baja California, 2 sites); and by coastal topography: protected (bays and coves, 7 sites in north, 3 in south), or exposed to the open ocean (7 sites in north, 5 sites in south). In Oregon, population structures at protected and exposed sites were significantly different (p = 0.001). At protected sites, a large percentage of individuals were pre-reproductive juveniles. At exposed sites, the population was weighted towards large individuals capable of high reproductive output with few juveniles present in the population. This pattern may reflect inter-annual variation in recruitment in Oregon, with protected areas experiencing more constant recruitment. In California and Baja California all populations were predominately composed of juveniles, regardless of coastal topography (p = 0.336). Differences in age-structure of the population create significant differences in relative reproductive potential. The potential reproductive output of a population was calculated by quantifying the eggs produced by snails covering the size range of reproductive individuals. Populations in the north at exposed sites produce eggs at a rate up to 2 orders of magnitude higher than populations in the south (p = 0.004). These variations in size-structure of C. funebralis populations with coastal topography and latitude may have important impacts for the population distribution. KEY WORDS: Population dynamics · Age structure · Chlorostoma funebralis · Recruitment Full text in pdf format PreviousNextCite this article as: Cooper EE, Shanks AL (2011) Latitude and coastline shape correlate with age-structure of Chlorostoma (Tegula) funebralis populations. Mar Ecol Prog Ser 424:133-143. https://doi.org/10.3354/meps08993 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 424. Online publication date: March 01, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Interannual variability in size structure, age, and growth of jumbo squid (Dosidicus gigas) assessed by modal progression analysis

Abstract Keyl, F., Argüelles, J., and Tafur, R. 2011. Interannual variability in size structure, age, and growth of jumbo squid (Dosidicus gigas) assessed by modal progression analysis. – ICES Journal of Marine Science, 68: . A range of growth rates, longevity, maximum size, and number of annual cohorts have been documented for jumbo squid (Dosidicus gigas). Genetic and phenotypic plasticity have been mentioned as possible sources of the differences in maximum sizes. Here, a large dataset on length frequencies derived from the industrial jig fishery for the species off Peru from 1991 to 2007 was corrected for observation bias introduced by the fishing gear and used to analyse growth-related parameters by modal progression analysis (MPA). Mean growth rates of 33 cohorts varied interannually (11–44 mm month−1), as did longevity (11.1–32.1 months) and mean maximum size (273–1024 mm). Intra-annual difference in growth parameters was not significant. The number of cohorts per year fluctuated between 0 and 6. Fast-growing cohorts with medium longevity and large terminal size were found during moderately cool periods, and long-lived, slow-growing cohorts with small terminal size during extreme ecosystem conditions (El Niño and La Niña). The application of MPA to cephalopod populations has been rejected before, but the results presented suggest that a modified approach is appropriate if it allows for varying growth parameters and includes a correction for observation bias.

Trichodesmium spp. population structure in the eastern North Atlantic subtropical gyre

Trichodesmium spp. population structure was studied based on a transect in the eastern subtropical North Atlantic in October 2006, covering a gradient from near-temperate conditions up to fully subtropical, oligotrophic waters. Trichodesmium spp. trichomes were counted and measured, distinguishing free trichomes from those forming macroscopic colonies (“puffs” or “tufts”). Moreover, for both free and colonial individuals, abundance and size structure variation were quantified from single trichome-level data. Nitrogen fixation rates were estimated using an empirical approach based on abundance observations and a heuristic approach to incorporate variation in trichome size and environmental temperature. A gradual change towards greater sizes and increased abundances was observed as physical conditions became subtropical. An extreme response was detected at one station (29.8°W, 26.3°N), in which both in situ and remote sensing data revealed the presence of a Trichodesmium bloom. Despite the presence of a mesoscale anticyclonic eddy, a close examination of prior surface conditions using altimeter data indicated that the bloom was related to the advection of southern waters. In general, our results highlighted the importance of free trichomes in Trichodesmium spp. populations in this part of the North Atlantic. Furthermore, they suggested a possible role of both size and population structure on the wide range of N 2 fixation estimates currently available in the literature.