The generalization of Gulland’s method: how to estimate maturity ogives when juvenile data are missing while spawner demography is known

The proportions of mature individuals at age or length, collectively known as the maturity ogive,are a key population characteristic and serve as critical input to age-disaggregated stock assessments. John Gulland showed in 1964 that it is possible to estimate maturity ogives even when representative data on immature individuals are not available, provided that one can distinguish newly mature individuals (first-time spawners) from those that had matured earlier (repeat spawners). Gulland’s method offers a valuable tool for obtaining information on an unobserved part of a population and is also applicable to other ontogenetic transitions, such as metamorphosis, smolting, ontogenetic niche shifts, and sex change. Here we present a full derivation of Gulland’s method from first principles, applicable to the general case in which the survival of immature, first-spawning, and repeat-spawning individuals may differ. Better observation methods, in particular in sclerochronology and histology, are expected to make meeting this method’s data requirements—i.e., the separation of first-time and repeat spawners—more often achievable, and estimating maturity ogives could serve as an additional incentive for allocating resources to enhanced data collection. With the generalization presented here, we hope to make Gulland’s method better known and more widely accessible

Report from the Expert Panel on the evaluation of the VRZs during the 2018/19 fishing season

In July 2018 ARK (the Association of Responsible Krill harvesting companies) launched a set of voluntary measures, known as ARK’s Commitment, which were proposed to improve the long-term sustainability of the krill fishery. The Commitment was initiated with support from Greenpeace, WWF and The Pew Charitable Trusts as a precautionary action whilst CCAMLR developed spatial management of the krill fishery in Area 48. The Commitment, which took the form of Voluntary Restriction Zones (VRZs), was implemented for the 2018-19 fishing season. The krill fishing fleet associated with ARK agreed to avoid fishing in an area of up to 40 km from penguin colonies in Subarea 48.1 during the penguin breeding season

Report from the Expert Panel (EP) on the evaluation of the ARK VRZ commitment during the 2019/20 fishing season

This is an update on the operation of and compliance with the ARK commitment during its second year of operation. The evaluation was completed by an expanded EP compared to 2019 (Appendix 1) and based on communication through video conferences and e-mails. The EP had access to fishery data up to the end of July 2020 and based its evaluation on this information combined with CCAMLR reports and recent publications related to the Antarctic ecosystem

Standing stock of Antarctic krill (Euphausia superba Dana, 1850) (Euphausiacea) in the Southwest Atlantic sector of the Southern Ocean, 2018–19

Estimates of the distribution and density of Antarctic krill (Euphausia superba Dana, 1850) were derived from a large-scale survey conducted during the austral summer in the Southwest Atlantic sector of the Southern Ocean and across the Scotia Sea in 2018–19, the ‘2018–19 Area 48 Survey’. Survey vessels were provided by Norway, the Association of Responsible Krill harvesting companies and Aker BioMarine AS, the United Kingdom, Ukraine, Republic of Korea, and China. Survey design followed the transects of the Commission for the Conservation of Antarctic Marine Living Resources synoptic survey, carried out in 2000 and from regular national surveys performed in the South Atlantic sector by the U.S., China, Republic of Korea, Norway, and the U.K. The 2018–19 Area 48 Survey represents only the second large-scale survey performed in the area and this joint effort resulted in the largest ever total transect line (19,500 km) coverage carried out as one single exercise in the Southern Ocean. We delineated and integrated acoustic backscatter arising from krill swarms to produce distribution maps of krill areal biomass density and standing stock (biomass) estimates. Krill standing stock for the Area 48 was estimated to be 62.6 megatonnes (mean density of 30 g m–2 over 2 million km2) with a sampling coefficient variation of 13%. The highest mean krill densities were found in the South Orkney Islands stratum (93.2 g m–2) and the lowest in the South Georgia Island stratum (6.4 g m–2). The krill densities across the strata compared to those found during the previous survey indicate some regional differences in distribution and biomass. It is currently not possible to assign any such differences or lack of differences between the two survey datasets to longer term trends in the environment, krill stocks or fishing pressure

Cabled ocean observatory data reveal food supply mechanisms to a cold-water coral reef

We investigated food supply mechanisms to a cold-water coral (CWC) reef at 260 m depth on the Norwegian continental shelf using data from a cabled ocean observatory equipped with Acoustic Doppler Current Profilers (ADCPs), an echosounder, and sensors for chlorophyll, turbidity and hydrography in the benthic boundary layer (BBL). Tidal currents of up to tens of cm s−1 dominated BBL hydrodynamics while residual currents were weak (∼10 cm s−1), emphasizing a supply and high retention of locally produced phytodetritus within the trough. A direct connection between the reefs and surface organic matter (OM) was established by turbulent mixing and passive particle settling, but relative contributions varied seasonally. Fresh OM from a spring-bloom was quickly mixed into the BBL, but temperature stratification in summer reduced the surface-to-bottom connectivity and reduced the phytodetritus supply. A qualitative comparison among acoustic backscatter in the ADCPs (600 kHz, 190 kHz) and echosounder (70 kHz) suggests that vertically migrating zooplankton may present an alternative food source in summer. Nocturnal feeding by zooplankton in the upper water column sustains downward OM transport independent from water column mixing and may dominate as food supply pathway over sedimentation of the phytodetritus, especially during stratified conditions. In addition, it could present a concentrating mechanism for nutritional components as compensation for the deteriorating phytodetritus quality. Overall, the observed patterns suggest seasonal changes in the food supply pathways to the reef communities. The moderating role of temperature stratification in phytodetritus transport suggests stronger dependence of the cold-water corals on zooplankton for their dietary requirements with increased stratification under future climate scenarios. This study demonstrates the added value of permanent ocean observatories to research based on dedicated campaigns and regular monitoring

Effects of turbidity on the spontaneous and prey-searching activity of juvenile Atlantic cod (Gadus morhua)

Increasing turbidity in coastal waters in the North Atlantic and adjacent seas has raised concerns about impacts on Atlantic cod (Gadus morhua) using these areas as nurseries. A previous experiment (Meager et al. 2005 Can. J. Fish. Aquat. Sci. 62, 1978–1984) has shown that turbidity (up to 28 beam attenuation m−1) had little effect on the foraging rate of juvenile cod. Although this was attributed to cod using chemoreception in conjunction with vision to locate prey, foraging rates may also be maintained by increased activity. Higher activity, however, is energetically costly and may offset benefits from increased foraging return