Assessing the risk to aquaculture from Harmful Algal Blooms (HABs)

Abstract

Harmful Algal Blooms (HABs) occur worldwide with regionally based differences in their
occurrences. Some of these events have human health consequences due to associated
shellfish poisoning symptoms after consumption of contaminated organisms. Negative
economic implications from HABs also occur for shellfish and finfish aquaculture associated
industries and coastal communities. Understanding temporal and spatial patterns of HABs in
coastal areas of Scotland is required to support the aquaculture industry and its insurers. The
use of long-term data series comprising phytoplankton data and extrinsic variables
(climatological, environmental, fish farm biomass) was undertaken to identify countrywide
patterns and assess the relationships that improve the understanding of HAB ecological
dynamics. A range of statistical approaches including descriptive analyses, spatial statistics
and linear models were used to analyse time series of HAB frequency or cell abundance of the
main taxa related to shellfish poisoning (Dinophysis, Alexandrium, Pseudo-nitzschia) and
farmed fish kills (Karenia mikimotoi) in Scotland.
Interannual frequency of HAB occurrence across the time demonstrated there is no increasing
trend in Scottish coastal waters. Seasonality was identified as a crucial component in
regulating the occurrence of HABs, also determining timing, location and scale of blooms
across the country and on a genera specific basis. The non-correlation between phytoplankton
and biotoxin groups underlines their complex interaction and implies different factors might
act to regulate these dynamics. The use of genera level constrains this assessment which
encourages further study at the species level. The low spatial connectivity found in HAB
events, and the diverse temporal-spatial patterns suggested annual and local conditions are
important in regulating these dynamics. The varied relationship between geographical areas
and seasons suggested local conditions play an important role in regulating HABs in Scotland.
Important environmental variables identified were sea surface temperature and wind intensity.
These presented non-linearity in specific study areas and explained a proportion of cell
response. The highest risk of blooms occur during summer months, with the Shetland Islands generally experiencing a higher frequency in comparison with other areas. Assessing the
relationship of farmed fish biomass (proxy for nutrient loading in the water column) indicated
a lack of relationship with taxa impacting shellfish farms and human health, and Karenia
mikimotoi affecting farmed fish. The production of a risk index including ‘hazard’, ‘exposure’
and ‘vulnerability’ components was useful to estimate the overall risk and assess the
contribution of each component in a socio-economic context, both globally and within a
Scotland based case study. The highest socio-economic risk was identified in the Shetland
Islands, Orkney and Outer Hebrides, mainly related to high dependability on fish farming of
the population in the Islands.
These findings highlight the complex interaction of blooms with extrinsic variables,
suggesting local conditions or stochastic mechanisms are likely relevant in explaining HAB
ecological mechanisms. These results support the robust and localised monitoring of
phytoplankton and biotoxins that pose a threat for the development of the Scottish aquaculture
industry, indicating useful sites for preliminary warnings.

Date of Award	6 Feb 2023
Original language	English
Awarding Institution	University of the Highlands and Islands
Sponsors	AXA-XL
Supervisor	Keith Davidson (Supervisor) & Michael Burrows (Supervisor)