The Evolution of Alternative Buoyancy Mechanisms in Freshwater Fish Eggs

Alternative reproductive tactics (ARTs) are behavioural, morphological, and physiological traits associated with alternative reproductive phenotypes within a population or species. ARTs are widespread in nature, and are a particular feature of teleost fishes. However, few studies have examined egg buoyancy mechanisms in the context of the evolution of ARTs in freshwater fishes. In marine fishes, egg buoyancy is achieved chiefly through hydration. While the buoyancy of freshwater fish eggs has been suggested to be determined primarily through the presence of oil droplets, the majority (60%) of freshwater pelagic eggs do not possess an oil droplet. We applied a physical model of buoyancy to understand the contributions of oil droplets and hydration to the buoyancy of pelagic freshwater fish eggs. We further used phylogenetic regression to estimate the effect of the relative size of the perivitelline space, habitat and parental care on the occurrence of oil droplets, while controlling for non-independence among species due to phylogenetic relatedness. Our analysis demonstrates that the probability of oil droplets in freshwater pelagic eggs exhibits a significant negative relationship with the size of perivitelline space, which may reflect a trade-off relating to energy allocation in contrasting habitats. We also demonstrate a positive association between the probability of oil droplets and the provision of parental care and occupancy of lentic habitats. These findings indicates the evolution of contrasting buoyancy mechanisms as novel ARTs in freshwater fishes. A theoretical model in combination with empirical analysis indicate the evolution of novel ARTs in freshwater fishes as adaptive responses to flow conditions.

Larval Fish Assemblage Structure at Coastal Fronts and the Influence of Environmental Variability

Within the coastal zone, oceanographic features, such as fronts, can have major effects on the abundance and distribution of larval fish. We investigated the effects of fronts on larval fish assemblages by jointly collecting physical (ADCP and CTD) and biological (larvae) data in the nearshore waters of the south coast of South Africa, on four separate neap-tide occasions. Accumulation of fish larvae at predominantly internal wave-associated fronts was observed, with higher larval densities inshore of and within the front than farther offshore. On each occasion, larvae of coastal species with pelagic eggs (Mugillidae and Sparidae) were numerically dominant at the front itself, while inshore of the front, larvae of coastal species with benthic eggs (Gobiesocidae and Gobiidae) were more abundant. Offshore catches mainly comprised Engraulidae (pelagic species with pelagic eggs) larvae, which were generally restricted to the bottom, where current velocities were onshore on each occasion. On the occasion when fast (>100 cm/s) currents prevailed, however, accumulation of the larvae of coastal species occurred offshore of the front, and larvae were mixed throughout the water column. Thus, larval occurrence at these coastal frontal systems was strongly affected by the degree of mixing by currents, which on most occasions resulted in onshore retention. The results underline the importance of frontal systems in determining the nearshore distributions of fish larvae, particularly by retaining coastal fish species in the inshore region. The environmental variability observed at these frontal systems has potential implications for larval connectivity of fish populations.

The making of a genetic cline: introgression of oceanic genes into coastal cod populations in the North East Atlantic

Coastal Atlantic cod (Gadus morhua) in the Northeast Atlantic has seen a continuous decline since the industrialization of the coastal fishery and management needs to address the spatial and temporal complexities of coexisting cod stocks. Toward that end, genetic analyses and oceanographic modelling of coastal and oceanic cod larval drift patterns were combined to elucidate the mechanisms responsible for an observed genetic cline over a >1500 km stretch along the coast of Norway. The results indicate that the north-south cline in coastal cod represents an extended contact zone between genetically divergent North Sea and North East Arctic cod and is maintained by two-way gene flow: by northward drift of pelagic eggs and larvae and by southward spawning migrations of North East Arctic cod. Computer simulations verify that the genetic cline can be established rapidly if gene flow into coastal populations is substantial. The shape of the cline, on the other hand, was found to be largely insensitive to the total amount of gene flow and therefore carries little information on extent of gene flow into and among coastal populations.

How many generations does Epischura baikalensis (Copepoda, Calanoida) have in Lake Baikal?

Epischura baikalensis Sars is a dominant pelagic species of the Lake Baikal zooplankton. We empirically determined the duration of the development of Epischura from field data, as it is not possible to determine this experimentally. Based on our data, we conclude that the central pelagic food web of metazoan species in Lake Baikal has not two, as was previously accepted, but three generations during a year and each lasts not 6 but instead 3 to 4 months. The life cycle of the species also possibly includes a 3-months long winter embryonic diapause, starting in November and ending in late February with the start of the under-ice algae bloom. The high nauplii density in spring is not the result of high adult density, suggesting the presence of resting eggs in the E. baikalensis life cycle. We also found a large morphological difference between the pelagic eggs of the species in the winter and in the summertime. The possible participation of diatom algal toxins in the regulation of the seasonal dynamics of their main consumer in Lake Baikal, similar to those known in marine ecosystems, is discussed. The population dynamics in a year with relatively low diatom algae density show four peaks of reproduction, i.e., in February, June, September, and December. In years with significant (more than 15 000 cells/l) diatom blooming we found mass mortality of E. baikalensis juveniles in the plankton, with only two clear peaks of nauplii density. A negative correlation was found between the number of population density peaks for E. baikalensis and the spring-blooming of diatom algae.

Novel fisheries investigations by Harald Dannevig: some parallels with Johan Hjort on the other side of the world

Harald Dannevig was Australia’s first Director of fisheries research and Director of Australia’s first ocean-going research vessel. Dannevig’s initial contributions concerned hatchery technology, freshwater fisheries, and impacts of estuarine prawn trawling. Later, he revealed the growth and migration of sea mullet, the spawning of pelagic eggs in the coastal ocean, and he was the first to demonstrate the effect of onshore winds on recruitment to estuarine fisheries. Using plans of the first Norwegian research trawler Michael Sars, he advised on the construction and commissioning of Endeavour. He organized 99 research voyages over 6 years to determine suitable trawling grounds over ∼7000 km, discovering 263 new species, including 96 new fish species and ∼5000 catalogued specimens. Harald Dannevig’s significant achievements in Australia were soon forgotten after his death with the loss of Endeavour in the Southern Ocean at the beginning of World War I. Both Johan Hjort and Dannevig were numerate, loved natural history, and were keenly observant on the deck. As these two scientists did not correspond, their innovative and parallel thinking stems from the shared university environment with G.O. Sars, and the rapport between Sars and Harald’s father Gunder Dannevig, concerning the fish hatchery and stocking of larval cod.

Survival and dispersal variability of pelagic eggs and yolk-sac larvae of central and eastern baltic flounder (Platichthys flesus): application of biophysical models

A hydrodynamic model coupled with a Lagrangian particle tracking technique was utilized to simulate spatially and temporally resolved long-term environmentally related (i) size of habitat suitable for reproduction, (ii) egg/yolk-sac larval survival, (iii) separation of causes of mortality, and (iv) connectivity between spawning areas of Baltic flounder with pelagic eggs. Information on reproduction habitat requirements and mortality sources were obtained from field or laboratory studies. In our modelling study we only quantified physical processes generating heterogeneity in spatial distribution of eggs and yolk-sac larvae, as e.g. predation is not accounted for. The spatial extent of eggs and larvae represented as modelled particles is primarily determined by oxygen and salinity conditions. The reproduction habitat most suitable was determined for the Gdansk Deep, followed by the Bornholm Basin. Relatively low habitat suitability was obtained for the Arkona Basin and the Gotland Basin. The model runs also showed yolk-sac larval survival to be to a large extent affected by sedimentation. Eggs initially released in the Arkona Basin and Bornholm Basin are strongly affected by sedimentation compared with those released in the Gdansk Deep and Gotland Basin. Highest relative survival of eggs occurred in the Gdansk Deep and in the Bornholm Basin. Relatively low survival rates in the Gotland Basin were attributable to oxygen-dependent mortality. Oxygen content had almost no impact on survival in the Arkona Basin. For all spawning areas mortality caused by lethally low temperatures was only evident after severe winters. Buoyancy of eggs and yolk-sac larvae in relation to topographic features appear as a barrier for the transport of eggs and yolk-sac larvae and potentially limits the connectivity of early life stages between the different spawning areas.