“The red tide does not discriminate.” This logical statement is from Homer, a local spear fisherman, living and fishing in La Paz for the last 19 years. Homer is the one who ultimately satisfied my curiosity about why thousands of sardines ended up dead in the bay of La Paz on July 15, 2024.
My friend asked me if I had witnessed the dead fish phenomenon, to which I answered, not visually, but definitely via olfactory! I was on site by 11 AM on that day and the cleaning crew had already bagged stacks of approximately 5 tons of organic waste, all of which mysteriously happened to be all dead sardines.
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“Why just sardines?” was the question that instantly popped into my head that day. What could be the reason/s that led this specific variety of fish to die so suddenly, with no other precedent?
The initial speculations pointed towards either high temperatures or the red tide. But as Homer pointed out: “The red tide does not discriminate.”
To his point, if it was the Red Tide, there would have been other varieties along with the sardines dead on the beach. A possibility that eventually got ruled out by the results of the microscopic analyses carried out by the State Commission for Protection against Health Risks (COEPRIS) of seawater samples taken in the Bay of La Paz.
A red tide is Harmful Algal Bloom or HAB and involves a rapid multiplication of specific types of microscopic algae in the water. Which when blooming may produce toxins that kill fish and shellfish or make them unsafe for consumption. Additionally, red tide may cause oxygen depletion suffocating all types of marine life, not limited to just sardines.
While the COEPRIS agency reported that it will continue to investigate the causes of the phenomenon, I decided to do my own research.
With the red tide ruled out, I approached Homer, a local spear fisherman, for his guess on plausible causes. Homer moved to La Paz nineteen years ago and has been fishing in the bay since then. He offered me an interesting perspective based on his experience.
This year, he observed a two-week delay in the usual warming pattern of the Gulf of California, leading to a recent thermocline formation. This delay created a rapid formation of the thermocline that might have created a significant temperature difference between the surface layer and the cooler water below.
What is a thermocline and why is it relevant here you might ask?
The thermocline is a vital layer within large bodies of water like oceans and lakes. It acts as a thermal barrier, separating the warm surface water from the much colder deep water. This plays a crucial role in the marine ecosystem as many species prefer specific temperature ranges and tend to stay within certain depths relative to the thermocline. Plankton distribution, which forms the base of the ocean food chain, is also influenced by it.
To Homer’s reflections, he believes the sardines were in their typical location, likely following food sources indicated by birds. He suggests the sardines didn’t have enough time to adjust to the colder water below the thermocline, resulting in their death.
A delayed thermocline formation is plausible. Warmer air and water temperatures due to climate change can influence the timing and intensity of thermocline development.
Sardines are known to be schooling fish that tend to stay within specific temperature ranges. They might follow food sources like plankton blooms that occur at certain depths. A rapid temperature change, especially if the thermocline formed very close to their usual depth, could stress the sardines.
Sudden temperature fluctuations can disrupt them and lead to death.
If the thermocline formed quickly the way Homer explained it, the sardines would have been trapped above it due to food availability or limited awareness, they might not have been able to reach the cooler water below in time.
Homer also shared an alternative theory. A thermocline could potentially trap plankton at a certain depth, attracting fish like sardines feeding on the concentrated food.This scenario might also explain why some other fish species weren’t affected – they might not have been targeting the same plankton layer as the sardines.
If the thermocline formed close to the sardines’ usual depth, they might have been trapped in warmer water with limited escape options. Other fish species that typically inhabit deeper or shallower waters might have been less affected by the temperature shift.
As one local marine biologist pointed out to me: “Impossible to know the cause without extensive study. Sometimes schools of sardines can follow plankton into the shallows and becomes trapped, they could be chased to exhaustion by prey and then get washed up, or it could be related to a temperature shock amongst other possibilities”
We might never know the real cause and that’s okay. Homer points out, no real harm was caused here. In reality the dead sardines have created a surplus of fertiliser for the plankton, creating a surplus of plankton growth. This in turn will provide additional food sources and protection to many species.
Homer concluded saying that he will personally wait for a two weeks window before getting in the water again and he recommends others to do the same to avoid contaminants. – Naailah Auladin, La Paz Instagram: @naailahauladin
