20 JUNE 2022 • WORLD AQUACULTURE • WWW.WA S .ORG Currently almost every state in the US experiences some kind of HAB event and, with climate change, these blooms are predicted to increase in both frequency and severity. This increase in HABs, along with the push to move aquaculture facilities farther offshore, creates a need for remote sensing technologies that can monitor effectively at these offshore locations. The growing interest to move large-scale aquaculture operations farther offshore requires creative solutions to address the challenges of the harsh and/or exposed environment. Daily monitoring of net pen sites, including for HABs, becomes increasingly challenging. Additionally, increased dissolved nutrient inputs, such as uneaten feed and fish waste have potential to affect phytoplankton prevalence, another reason increased monitoring is so important in these areas. However, there are currently no commercial offshore aquaculture facilities operating in U.S federal waters; therefore, further research is required to better understand the potential correlation between increased phytoplankton and the prevalence of offshore aquaculture facilities. As noted above, this perceived increase may be simply related to intensified monitoring efforts, indicating that blooms existed naturally without additional nutrient input. Warming ocean temperatures, along with increased terrestrial nutrient input from runoff, including fertilizers and sewage are the most documented and studied causes of HABs. Additionally, not all algal blooms are harmful. Blooms can sometimes be good indicators of environmental change and also provide a rich food source for a multitude of aquatic species. Often, HABs are detected after the bloom has already occurred, limiting the effectiveness of management efforts. Remote Sensing Technologies Remote sensing is defined by the National Aeronautics and Space Administration (NASA) as “the process of gathering information about something without touching it.” The power of remote sensing lies in its ability to provide spatial and temporal views of surface water quality and atmospheric parameters Harmful algal blooms (HABs) consist of aggregations of algae that range in size from microscopic single-celled organisms (microalgae) to large seaweeds (macroalgae). HABs can have lethal and sub-lethal effects on aquatic life, including deoxygenation (caused by the depletion of oxygen via the bacterial decomposition of algae), ichthyotoxicity (from phycotoxins produced), or physical interference (clogging or damaging of gills), emphasizing the importance of HABmonitoring at or near these sites. The HABs that garner the greatest concern are those that can produce high levels of toxins and those that create enough biomass to deplete oxygen levels and lead to the suffocation and death of aquatic life. There are over 5,000 species of phytoplankton but fewer than 80 of those are known to be toxic (National Research Council 1999). This article pays special attention to those toxic species that can cause illness and death to humans and marine life and those that cause an array of economic impacts to aquaculture. Estimates of actual impacts are few, in part because these economic losses are difficult to quantify. However, on a global scale, HABs result in approximately $8 billion/year of losses because of mass mortalities of finfish, shellfish harvesting bans due to accumulation of phycotoxins and human health issues. According to the Harmful Algal Event Database (HAEDAT), a Meta database containing records of HABs dating back to 1985, there is a significant increasing trend in all HAB events globally. Part of this observed HAB expansion simply reflects a better realization of the true or historic scale of the problem, long obscured by inadequate monitoring. Other contributing factors include the dispersal of species to new areas, the discovery of newHAB poisoning syndromes or impacts, and the stimulatory effects of human activities like nutrient pollution, aquaculture expansion and ocean warming, among others (Anderson et al. 2021). The continued diversification of harmful phytoplankton species and toxins represents a growing challenge to resource managers in terms of monitoring and management; therefore, trends need to be considered regionally and at the species level moving forward. Remote Sensing Technologies to Monitor Harmful Algal Blooms in Offshore Aquaculture Devan Nichols and Timothy Hogan FIGURE 1. Various sensors aboard satellites that are used for monitoring HABs globally. Clockwise from top left: a MODIS sensor onboard the Terra satellite (en.wikipedia.org/wiki/Terra_(satellite), top right: a OLCI sensor onboard the Sentinel 3 satellite (en.wikipedia.org/wiki/Sentinel-3 ), bottom: a VIIRS sensor onboard the JPSS satellite (commons.wikimedia.org/wiki/ File:NOAA-20_ JPSS-1_spacecraft_model_1.png).
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