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9.0 DIATOM BLOOMS AND FISH MORTALITY

Excluding the toxic diatom genus Pseudo-nitzschia whose toxin, domoic acid, causes ASP and mortality of birds and marine mammals was discussed in Section 2.0, diatoms usually are not included among harmful phytoplankton groups. However, diatoms frequently have been implicated in causing mortality and physiological impairment of farmed fish, adverse effects most often related to morphological features of diatoms or to their abundance; i.e. non-toxicological stressors. Fourteen diatom species have been reported to be harmful through such effects (Table 8).

Table 7. Ecophysiological contrasts between naked cell and siliceous stages of the silicoflagellate Dictyocha speculum.

⁺ optimum, in culture

Unusual mortality of lingcod ( Ophiodon elongatus) kept in livewells prior to marketing led to the discovery that diatoms could be physically harmful to fish (Bell, 1961). Examination of the detrital layer that covered the discoloured lingcod gills revealed an abundance of a chain-forming diatom, Chaetoceros convolutus, whose barbed, siliceous spines (setae) penetrated into the gill tissue and caused histological damage. Bioassays confirmed that exposure of lingcod to dense concentrations of C. convolutus resulted in death caused by the penetration of the chaetocerid spines into the gills. Mortality of farmed fish attributable to spinous diatom blooms has accompanied the expansion of fish farming. Major dieoffs of pen-reared salmon as a result of gill piercing by spinous Chaetoceros concavicornis and C. convolutus are common in the Pacific Northwest of Canada and the U.S. (Albright et al., 1992, 1993; Harrison et al., 1993; Horner and Postel, 1993; Horner et al., 1990; Haigh and Taylor, 1990; Rensel et al., 1989; Rensel, 1993; Taylor, 1993); Taylor et al. (1994). In Chile, mortality of farmed salmon and sea trout occurred during a C. convolutus bloom (Clement and Lembeye, 1993). In Scotland, blooms of Chaetoceros debilis and Chaetoceros wighami in the Shetlands led to farmed salmon mortality (Bruno et al., 1989). And, recently, a 4-week bloom of C. wighami in Sea Loch, west Scotland led to inappetance, lethargy and mortality of Atlantic salmon resulting in a production loss of 170 t valued at £ 408,000 (Treasurer et al., 2003). Mortality of farmed coho salmon occurred during a bloom of the non-colonial species Corethron criophilum (Speare et al., 1989), characterized by two prominent apical spines rather than the much longer setae (n=4) found in chaetocerid cells. The morphological feature distinguishing these harmful diatoms from others (Table 8) is the presence of silicified setae or spines.

When gills are pierced and penetrated by spinous diatoms, the epithelium becomes physically irritated, forms lesions and produces excessive mucus that lead to asphyxiation (Kent et al., 1995; Yang and Albright, 1992). A toxin is not involved; mortality is mechanically induced via histological disintegration. Rensel (1993) reported that mortality can also occur without the penetration of spines into the gill filaments. Atlantic salmon exposed to about 0.5 million cells L ^-1 of C. concavicornis produced mucus in reaction to the accumulation of this diatom within interlamellar spaces. The setae did not then penetrate into gill tissue. Blood gas measurements revealed blood hypoxia developed, which Rensel interpreted to be the cause of mortality. It is unknown whether blood hypoxia might also explain the puzzling mortality of farmed salmon reported to occur during blooms of the non-spinous, colonial diatom species Leptocylindrus minimus (Clement and Lembeye, 1993) and Skeletonema costatum, Thalassiosira aestivalis and Thalassiosira rotula (Kent et al., 1995). Blooms of the gelatinous and colonial Chaetoceros socialis in Chilean waters were reported to have " affected fish appetite " on occasion (Clement and Lembeye, 1993). However, such inappetance would appear to be a transient and short-lived response given bloom dynamics and fish metabolism, and would not generally account for the harmful impact of spine-free diatoms.

The harmful diatom species (Table 8) produce blooms that are common and natural events, rather than dependent on fish farm stimulation. Their inimical effect on pen-reared fish occurs because aquacultural confinement prevents these fish from undertaking avoidance reactions needed to escape the threats of gill piercing and clogging posed by diatoms. Vagile, natural fish stocks do not appear to suffer diatom induced mortality or impairment. [However, episodes of this are unlikely to be detected.] While diatom induced mortality of farmed fish is primarily an artifact of pen-rearing, fish farm operations possibly aggravate the impact through selective stimulation of the naturally occurring, local bloom flora. Weak support for this possiblity was provided by Smith et al. (2001) who carried out a 10-year study of the population dynamics of the diatom Thalassiosira nordenskioeldii at a salmonid fish farm site. They claimed that an " increased contribution" of this diatom occurred, promoted by the long-term excretion of nutrients from the fish farm. The issue of the role of fish farm operations in altering phytoplankton community behavior is treated in greater detail in Section 10.

The harmful diatom species recognized and their blooms, possibly excluding Corethron criophilum, are common in coastal waters, including in Scotland. Blooms of non-spinous, chain-forming Cerataulina pelagica are also common in Scottish sea-lochs (Gamble et al., 1977; Hannah and Boney, 1983; Tett and Wallis, 1978; Wood et al., 1973). In a Loch Ewe experiment, C. pelagica bloomed in nutrient enriched and unenriched mesocosms (Gamble et al., 1977). This species is of interest because its blooms in New Zealand waters have caused fish kills both by gill clogging and from post-bloom anoxia (Taylor et al., 1985; Rhodes et al., 1993). This thinly silicified species is capable of growth at relatively low silicon concentrations during summer. Since silicon is not excreted at fish farms, C. pelagica is a leading candidate species if a diatom were to bloom in response to excreted nitrogen and phosphorus at those sites during summer in Scottish waters.

Shellfish are not only vulnerable to the accumulation of ASP-inducing domoic acid during Pseudo-nitzschia blooms, on which they filter-feed. Diatom blooms can also cause shellfish mortality or diminish their food quality. Blooms of the mucilaginous, colonial species, Thalassiosira mala, in Tokyo Bay clogged the gills of cultured bivalves resulting in a dieoff valued at 58 million ¥ (Takano, 1956). Shellfish mortality occurred during a C. pelagica bloom in New Zealand (Taylor et al., 1985). And a bloom of the diatom, Rhizosolenia chunii, in Australia was accompanied by an unpleasant, bitter taste of mussels, scallops and oysters, followed by high mortality that lasted 3 to 8 months after the bloom ceased (Parry et al., 1989).

Blooms of the diatom Coscinodiscus wailesii, a species that has become invasive and widely distributed in the North Sea, since first detected in UK waters off Plymouth in 1977 (Boalch and Harbour, 1977; Boalch, 1987), is a nuisance diatom in several unique respects. Its slime-producing blooms have affected fisheries by clogging fishermen nets which impairs catches during bottom dragging (Boalch, 1987) and pelagic trawling (Mahoney and Steimle, 1980). Boalch (1987) reports that trawlers have been lost when the weight of the netted slime became excessive. Coscinodiscus wailesii blooms also plague Japanese aquaculture. Its blooms take up the nitrogen (sometimes added as fertilizer) needed for Nori growth (edible seaweed); this reduces yields and can also cause hypoxia leading to faunal mortality (Manabe and Ishio, 1991). Blooms of other Coscinodiscus species commonly occur in the North Sea, where their oily secretions have caused bird mortality (Tåning, 1951). In the German Wadden Sea, large-scale anoxia and benthic faunal mortality have accompanied Coscinodiscus concinnus blooms (Delafontaine and Flemming,1997). The ecophysiological profile of C. wailesii (Rick and Dürselen, 1995; Nagai and Imai, 1999) suggests that Scottish sea lochs are suitable habitats for its blooms. Miyahara et al. (1996) have reported that " large scale occurrences of colony-forming diatoms of the genus Thalassiosira have caused major economic problems to fisheries".

In summary, although incidents of aquacultural mortality attributable to diatoms do not appear to have been reported from Scotland, most of the 14 species reported to be harmful, either because of their morphological features or bloom densities, occur in Scottish coastal waters. As a group, these species are much less worrisome than species of the toxic diatom genus Pseudo-nitzschia that bloom in these waters and contaminate shellfish with domoic acid responsible for ASP toxicity [Section 2.0].

Table 8. Diatoms reported to have had harmful effects on aquaculture and natural fisheries.

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