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As feed-additive aquaculture expands to open ocean areas, there is concern that ecologically important habitats may be adversely impacted by sedimentation of farm wastes. In this study, we investigated assimilation of salmon faecal wastes by an ecosystem engineering bivalve that occurs in open ocean environments (Atrina zelandica), as well as effects on physiology and fatty acid metabolism. A. zelandica were subjected to one of 3 treatment diets (fish faeces, 1:1 mix of algae:faeces and algae) in a 51 d laboratory trial. We found a diet-related response in fatty acid composition, including increased prevalence of oleic acid (OA) in digestive tissues of A. zelandica fed on both the fish faeces diet and the mixed diet, indicating fish wastes were assimilated in both treatments. Fish waste consumption was related to a more marked reduction in fatty acid content of digestive gland, as well as lower proportions of long-chain polyunsaturated fatty acids (LC-PUFA) in digestive tissues. Fatty acid composition in gonad and muscle tissues was more strongly influenced by sex. Regardless of dietary treatment, females accumulated C18 fatty acids in gonad tissues, particularly OA, which may preclude the use of OA as a fish waste tracer in this organ. The accumulation of specific fatty acids according to sex may indicate a capacity for preferential selection and retention or biosynthesis of biologically important fatty acids. If present, these mechanisms may increase resilience of A. zelandica to stress from deficiencies in LC-PUFA when using fish wastes as a trophic subsidy.

The black-foot abalone (pāua), Haliotis iris, is a unique and valuable species to New Zealand with cultural importance for Māori. Abalone are marine gastropods that can display a high level of phenotypic variation, including slow-growing or 'stunted' variants. This investigation focused on identifying factors that are associated with growth performance, with particular interest in the slow-growing variants. Tissue alterations in H. iris were examined using histopathological techniques, in relation to growth performance, contrasting populations classified by commercial harvesters as 'stunted' (i.e., slow-growing) and 'non-stunted' (i.e., fast-growing) from four sites around the Chatham Islands (New Zealand). Ten adults and 10 sub-adults were collected from each of the four sites and prepared for histological assessment of condition, tissue alterations, presence of food and presence of parasites. The gut epithelium connective tissue, digestive gland, gill lamellae and right kidney tissues all displayed signs of structural differences between the slow-growing and fast-growing populations. Overall, several factors appear to be correlated to growth performance. The individuals from slow-growing populations were observed to have more degraded macroalgal fragments in the midgut, increased numbers of ceroid granules in multiple tissues, as well as increased prevalence of birefringent mineral crystals and haplosporidian-like parasites in the right kidney. The histopathological approaches presented here complement anecdotal field observations of reduced seaweed availability and increased sand incursion at slow-growing sites, while providing an insight into the health of individual abalone and sub-populations. The approaches described here will ultimately help elucidate the drivers behind variable growth performance which, in turn, supports fisheries management decisions and future surveillance programs., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

Climate change associated temperature challenges pose a serious threat to the marine environment. Elevations in average sea surface temperatures are occurring and increasing frequency of marine heatwaves resulting in mortalities of organisms are being reported. In recent years, marine farmers have reported summer mass mortality events of the New Zealand Greenshell mussel, Perna canaliculus, during the summer months; however, the etiological agents have yet to be determined. To elucidate the role of thermal stress, adult P. canaliculus were exposed to three chronic temperature treatments: a benign control of 17 °C and stressful elevations of 21 °C and 24 °C. Eight mussels per treatment were collected each month throughout a 14-month challenge period to identify and investigate histopathological differences among P. canaliculus populations exposed to the three temperatures. Histopathology revealed several significant deleterious alterations to tissues associated with temperature and exposure time. Increasing temperature and progression of time resulted in 1) an increase in the number of focal lipofuscin-ceroid aggregations, 2) an increase in focal hemocytosis, 3) an increase in the thickness of the sub-epithelial layer of the intestinal tract and 4) a decreased energy reserve cell (glycogen) coverage in the mantle. Prolonged exposure, irrespective of temperature, impacted gametogenesis, which was effectively arrested. Furthermore, increased levels of the heat shock protein 70 kDa (HSP 70) were seen in gill and gonad from thermally challenged mussels. The occurrence of the parasite Perkinsus olseni at month 5 in the 24 °C treatment, and month 7 at 21 °C was unexpected and may have exacerbated the fore-mentioned tissue conditions. Prolonged exposure to stable thermal conditions therefore appears to impact P. canaliculus, tissues with implications for broodstock captivity. Mussels experiencing elevated, temperatures of 21 and 24 °C demonstrated more rapid pathological signs. This research provides further insight into the complex host-pathogen-environment interactions for P. canaliculus in response to prolonged elevated temperature., Competing Interests: Declaration of competing interest There are no conflicts of interests associated to this work., (Copyright © 2023 Elsevier Ltd. All rights reserved.)