Although gelatinous zooplankton are an important component of marine ecosystems, gelatinous mesozooplankton that are <2 cm are underrepresented in monitoring programmes. Here, the interannual variability of gelatinous mesozooplankton abundance and diversity was estimated from 167 zooplankton net samples that were collected in the Celtic Sea during seven fisheries surveys between 2007 and 2019 and analysed alongside environmental parameters. Compositional changes occurred interannually, including an overturn in the abundance ratio of two siphonophores (Muggiaea atlantica and Agalma elegans). Analysis of annual mean gelatinous abundance revealed no linear trend over time (Spearman, r = −0.09, p = 0.287); however, the interannual abundance varied by a factor of 33 (minimum mean abundance in 2013 = 7.36 ± 4.86 individuals m−3; maximum in 2017 = 244.82 ± 84.59 individuals m−3). Holoplanktonic taxa dominated the abundance of the gelatinous community (93.27%) and their abundance was negatively associated with summer sea surface temperature (represented by the 16°C isotherm in July), and the Eastern Atlantic Pattern index 3 months prior (April). Our data suggest that gelatinous mesozooplankton in the Celtic Sea may become less abundant with further ocean warming, and further highlight the need to monitor gelatinous mesozooplankton with a high taxonomic resolution moving forward.
To address population declines, many salmon populations are supplemented with hatchery-bred fish. Little is known about how growth performance of hatchery-bred and wild salmon compare during critical periods such as the first summer at sea. This study compares scale growth measurements between 1725 ranched and 1976 wild salmon from the Burrishoole catchment (western Ireland) over 62 years (1956–2017). Post-smolt growth (PSG) was greater in wild than ranched fish; this difference was consistent with compensatory growth rather than size-dependant growth. Temporal trends in marine growth of wild and ranched fish were similar, with a marked decline in PSG and return size after the early 2000s. The decline in PSG was significantly related to freshwater growth and autumn sea surface temperature in the Norwegian Sea. Temporal changes in the pattern of intercirculi spacings along the PSG growth trajectory were also identified; circuli deposited during the first summer at sea were narrower in the 2000s and 2010s compared to earlier decades, while circuli from the first winter were wider. The recent decline in growth affects both populations similarly, is associated with increasing temperature and is manifested as a change in growth pattern as well as overall growth.
Our understanding of the distribution and abundance of gelatinous mesozooplankton (0.2-20 mm) has been largely determined from the deployment of net sampling devices. Although zooplankton nets have been shown to underrepresent the abundance of certain gelatinous taxa in comparison to data from optical systems, nets still provide relative abundances for gelatinous zooplankton. A potential source of standardized, long-term, and broad-scale data for the relative abundance and distribution of gelatinous mesozooplankton is from fisheries monitoring programs which use zooplankton samplers such as the high-speed Gulf VII sampler to capture fish eggs and larvae in shelf and offshore waters. To evaluate the use of the Gulf VII sampler as a source of gelatinous mesozooplankton data, this study collected zooplankton samples using both a Gulf VII sampler and a vertical ring net at 15 stations off the northwest Irish and Scottish coasts in July 2017. The difference in the estimates of gelatinous mesozooplankton abundance and taxon richness of the Gulf VII and the ring net samples were not significant (mean abundance: Gulf VII = 135.7 AE 79.7 ind. m −3 , ring net = 149.7 AE 66.5 ind. m −3 ; taxon richness: Gulf VII = 15.2 AE 2.4, ring net = 15.4 AE 2.5), although the Gulf VII underestimated the abundance of the appendicularian Oikopleura sp. by a factor of 9.7 when compared to the ring net. Our findings describe how added value can be provided to already existing zooplankton monitoring programs while enhancing the temporal and spatial coverage of data available for gelatinous mesozooplankton.
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