Hydrozoan jellyfish and their interactions with Scottish salmon aquaculture
Abstract
Medusozoan jellyfish (Classes Scyphozoa and Hydrozoa) have gained a degree of worldwide
notoriety in the last fifteen years, particularly as anthropogenic influences such as climate
change and overfishing push some ecosystems toward their advantage (Lynam et al. 2005,
Purcell and Arai 2001, Purcell et al. 2007, Purcell 2012, Flynn et al. 2012, Dawson et al. 2014).
Accordingly, both the lay and scientific media have paid a good deal of attention to jellyfish
bloom phenomena and their impacts on human activities, but the bulk of this attention has
been devoted to larger, visually obvious species of Class Scyphozoa. Only recently have their
smaller cousins, the hydrozoans, come to be recognized as potentially problematic. This
thesis examines population ecology of hydrozoan medusae (hydromedusae) and their
implications for salmon aquaculture in Scotland.
My review of available literature has found hydrozoans to be a recognized – though under-
studied – problem for Scottish salmon (Chapter 1, Prospective monitoring of hydromedusa
populations at salmon aquaculture facilities). Typically, hydrozoan populations at salmon
farms have been discussed in the scientific literature only in the context of extremely dense
visible blooms or in the wake of major mortality incidents. This retrospective, rather than
prospective, approach has left a dearth of knowledge pertaining to hydromedusan
interactions with farmed fish, with both fish welfare and industry economics vulnerable to
future blooms.
This thesis sought to build a basis for the goals of prediction, avoidance, and mitigation of
harmful hydrozoan jellyfish blooms. First and foremost, this required the development of a
prospective time-series dataset of hydromedusan occurrences at salmon farms (Chapter 2,
Bacterial genera biodiversity in three medusozoan species in Shetland). To this end, four
farms were recruited as participants across a three-year survey. Weekly plankton tow-based
sampling at these sites identified which hydrozoan species could be expected to produce
blooms, the seasonality of such blooms, and the pathological sequelae that could be
expected in salmon after exposure to such blooms. Following one particularly dramatic
bloom, a spike in gill pathologies in salmon was observed, followed by a spike in overall
mortality and the eventual loss of up to £2.5 million value as the fish were humanely culled.
This survey also demonstrated that hydromedusan blooms are usually spatially and
temporally patchy, limiting the opportunities for geographically-encompassing predictive
power. Instead, individual aquaculture facilities may require site-specific risk assessment
and planning frameworks to monitor and cope with blooms. Potential methods for
continued basic monitoring and a mitigation strategy based on minimizing contact between
fish and high-density blooms are suggested.
A second mitigation goal examined the theory that medusae may act as vectors for microbial
pathogens, particularly Tenacibaculum maritimum (Ferguson et al. 2010, Delannoy et al.
2011; Chapter 3). Sampling methods designed to target T. maritimum were employed with
the aim of determining its distribution and role as a symbiont in various life stages of
medusozoan species. While T. maritimum itself was not observed, a number of other fish
pathogens were found in close association with several species. This included Aeromonas
salmonicida, known to cause furunculosis in aquaculture of both salmon and trout (Nomura
et al. 1992). Further work is required to piece together the nature of these associations.
Finally, Chapter 2 identified a particular hydrozoan genus, Obelia, as a likely significant
contributor to blooms at salmon aquaculture sites. One of its species, O. geniculata, has a
widely distributed and well-recognized benthic colonial life stage (called the hydroid stage)
in Scottish nearshore sublittoral environments. In attempting to sample these hydroids from
previously well-colonized sites in Shetland in late 2012, it became apparent that a severe
local reduction in the benthic population was taking place. This allowed for the opportunity
to study phylogeographic population structure – i.e. the boundaries of its gene pool(s) in
Scottish waters and its potential for dispersal during one seasonal reproductive period –
using a molecular study of the mitochondrial cytochrome oxidase subunit I (mtCOI) gene
(Chapter 4, Phylogeographic analysis of Obelia geniculata populations in the north of
Scotland). In sampling immediately after the observed dieback, O. geniculata was found to
follow a south-to-north pattern of genetic grouping, as well as a confirmed dieback.
However, this pattern disappeared in samples collected after the population had recovered,
probably due to the immigration of genetically novel individuals. This finding, in conjunction
with the spatial-temporal patchiness found in the medusa bloom stage, suggests the
importance of the larval stage as the primary stage for dispersal in the plankton. This study
was also able to compare present population genetic data with a set of O. geniculata mtCOI data collected between 1998 and 2002. The combined data potentially show a high degree
of mixing across a number of North Atlantic regions, including Icelandic and North American
sites. Further investigation will be required to discern whether this pattern is temporally
based (i.e. artefact of 15 years’ elapsed time in opportunities for population mixing), or
whether ecological, anthropogenic, or combined mechanisms are facilitating rapid transport
of propagules to yield a well-mixed population.
Further work in refining prediction and mitigation is still needed, as are effective veterinary
interventions in the event of blooms. Continued study into the ecological patterns of
colonization and dispersal may help to minimize exposure to blooms, by helping to assess
site-based risks. This research forms the basis for such studies into hydrozoan interactions
with salmon farms in Scotland, and how the industry might seek to minimize their impacts.
Type
Thesis, PhD Doctor of Philosophy
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