top of page

Gitga’at Harmful Algal Bloom (HAB) Monitoring

By Kim-Ly Thompson and Mary Reece


Harmful algal blooms (HABs) are blooms of plant-like plankton that can have harmful effects on seafood and people. Most species of phytoplankton are harmless and are in fact crucial for the health of the marine ecosystem. However, some species of phytoplankton produce toxins that can be harmful to humans and other mammals if they are consumed in high amounts.

For example, at high enough concentrations some Alexandrium species can cause paralytic shellfish poisoning, some species of Pseudo-nitzchia can cause amnesiac shellfish poisoning, and Dinophysis species can cause diarrhetic shellfish poisoning (Figure 1) .

These species of phytoplankton have existed along the Pacific coast for thousands of years and Gitga’at ancestors had many methods of making sure that people don’t get sick from eating shellfish, such as only harvesting during cold winter months. However, with waters warming due to climate change HABs are expected to last longer and be more frequent, which is why we need to pay close attention to these changes to make sure seafood consumers stay safe.

Since November 2021, the Gitga’at Oceans and Lands Department (GOLD) has partnered with the First Nation’s Health Authority’s (FNHA) We All Take Care of the Harvest (WATCH) project. WATCH’s primary purpose is to help communities access timely seafood safety information to help harvesters decide when and where their seafood is safe to harvest.

A first step is through HAB monitoring. Currently, water samples are collected from shellfish harvesting areas in Gitga’at territory once or twice a month (weather allowing) and are analyzed for the presence of harmful algae species and other environmental factors such as temperature and salinity. This monitoring complements biotoxin sampling done by the Guardians and can act as an early warning system and help to understand the trends and environmental drivers of biotoxins in shellfish.

Figure 1. A. Alexandrium spp. seen in samples from Union Pass, July 2022); B. Pseudo nitzchia spp. seen in samples from Old Town, September 2022; C. Dinophysis spp. seen in samples from Molsey Bay, January 2023. All photos by Mary Reece.

Monitoring results

A total of 261 samples have been analyzed from important shellfish harvest areas in the Territory since the WATCH program began (Table 1). Gitga’at Guardians and Mary Reece collect four water samples at each site, and Mary Reece brings these back to the office check for the presence and concentration of phytoplankton species in each sample.

Table 1. WATCH sampling effort 2022-2023. Each X represents one visit during which a grouping of samples taken from 1m, 5m, 10m and a tow sample were collected.

The most common potentially harmful phytoplankton species in Gitga’at waters since WATCH sampling began is Pseudo-Nitschia ssp (Figure 1, A). Pseudo-Nitzchia populations began to grow in June 2022 and was seen in very high concentrations at most sample sites by September.

The highest concentration of Pseudo-Nitzchia was seen at Big Bay in mid-September. By November 2022 and throughout the winter months, Pseudo-Nitzchia only appeared occasionally at low concentrations, however, an increase was noted at some sites in March and April 2023.

Alexandrium spp cells appeared in much lower concentrations than Pseudo-Nitzchia (Figure 1B), with concentrations increasing in some areas from May to July 2022, then occasionally appearing in low concentrations throughout the rest of 2022. No Alexandrium cells have been seen yet in 2023 samples.

Dinophysis cells appeared more consistently over time and across sites, but in concentrations at most ten times lower than pseudo-Nitzchia concentrations (Figure 1C). Blooms appeared to have formed and remained present from June through September 2022, with some low concentrations appearing at some sites in November 2022, and January and February 2023.

As of May 2023, concentrations of Dinophysis have increased to levels similar to the previous summer at once sample site (Old Town). Other species of phytoplankton that present some risk at high concentrations were also sometimes present in our samples (Figure 2).

The most common of these were Ceratium, Dictyocha and Chattonella. Dictyocha can be toxic to fish at high concentrations. It appeared consistently in most samples at low concentrations, with a slight increase at some sites from May through September 2022, then again in March 2023. Ceratium does not produce toxins but at very high concentrations has the potential to deplete oxygen and nutrients in an ecosystem.

The concentration of Ceratium at most sample sites increased in July and August 2022, but has remained low otherwise. Finally, there was a noted spike in Chattonella cells at Kishkosh and Old Town in May 2023.

At high concentrations Chattonella can be responsible for fish kills. Future sampling will reveal whether a bloom persists at those sites.

How does this relate to seafood safety?

The toxic effects of harmful algae species depend on many things including concentration, environmental variables, local oceanography, and possible interactions with other species.

For example, there may have been a connection between the presence of Alexandrium near Clamstown in mid- November and high levels of biotoxins in shellfish around the same time. However, even though no Alexandrium cells were detected in the water column at the end of January 2023, biotoxins were present. For the most part, harmful algae species were not present or were in very low concentrations during typical shellfish harvest season (late fall to early spring).

During the harvest season the Guardians sample shellfish from important harvest sites to have them analyzed for toxins in shellfish tissues. This is the most direct way to know whether shellfish are safe to eat or not.

As ocean conditions change it will be important to continue monitoring and understanding the links between what is happening in the environment, how algae species are responding, and ultimately whether there are repercussions for seafood toxicity.

Some steps towards this goal will be to synchronize phytoplankton monitoring with shellfish tissue testing, continue measuring local environmental variables such as water temperature and acidity, and finally continue working towards building a local seafood testing facility.


bottom of page