Fish kills have jumped to the front pages of newspapers across the country this summer. Â And while the harmful algal blooms (or HABs) across the Florida coast have captured the attention of the national media, we are having our own mini-explosion right here in southern California.
Our local interest piqued when our colleagues at The Nature Conservancy noticed a fish kill event in the waters feeding into our Ormond Beach Wetland site in late July. Â A subsequent fish kill in Malibu Lagoon 50 km (30 mi) to the south in late August got the attention of Los Angeles newsrooms.
Fish Kills
In both our Ormond and Malibu situations, a few dead fish were noticed floating on the surface and within a day or two many dozens to hundreds of fish started accumulating along the water’s edge and stinking up the surrounding area.  This is the point when my colleagues and I start getting questions and our local, state, and federal agencies start investigating.  Within another day or two the questions of “What happened?” and “What killed these fish?” and “Is this dangerous to us?” start coming fast and furious.
Dr. Mark Okihiro from California Fish and Wildlife performing a necropsy on a striped mullet from Ormond Beach in our Conservation Biology Laboratory at CSU Channel Islands on August 1, 2018.
Who is dying?
Generally, we’ve seen mostly fish dead, but we are occasionally seeing dead invertebrates as well.  Rarely do we do a systematic survey of the waterbody during a kill given the smells and other logistic challenges, but it is uncommon for one of these events to impact only a single species.  Several fish species were impacted both at Ormond and Malibu, including the usual small-bodied suspects in these systems such as mosquitofish (Gambusia affinis), arroyo chub (Gila orcuttii), and topsmel (Atherinops affinis).  But most of the current attention centers around the larger, more obvious striped mullet (Mugil cephalus) which have been accumulating by the scores.  Most of the mullet we’ve seen are around the foot to foot-and-a-half size range (the typical size for a full grown adult), but some of my colleagues are reporting meter-long (3 foot) dead mullet in Malibu Lagoon the weekend of the die off!
California State Parks working to clear thousand of dead fish from Malibu Lagoon. Image: LA Times
What is killing these fish?
We don’t know yet, but it seems to be related to the physical environment rather than poisoning.
The first worry when we see such a large die off of many individuals is a toxin.  Indeed, that is what is happening in Florida right now: a red tide plus phtyotoxins produced by very largescale HABs are killing hundreds of thousands of organisms up and down  both of their Gulf and Atlantic coasts.  We don’t think that the unfolding Florida scenario is the proximate explanation here in southern California.
Florida Red Tide Status as of September 7, 2018. Source: Florida Fish and Wildlife Commission.
While we have not yet gotten the chemical analyses from Malibu Lagoon fish, the fish we sampled from Ormond show no signs of pesticides or related toxins that might have poisoned the water in which these fish were living (as has been the case in previous kills in previous years).  A gross inspection of the internal and external anatomy of fish from our impacted Ormond area found no obvious lesions or evidence of chemical toxicity.  So our Ormond fish kill doesn’t appear to have been acute poisoning from a chemical toxin.
This leaves us with the next likely candidate: low oxygen and/or high heat stress. Â At the moment this is the leading candidate for both the Ormond and Malibu events. Â While we did not have our Ormond site instrumented, spot checks of the oxygen levels during daylight hours show nothing particularly out of the ordinary. Â Malibu Lagoon has a much more robust monitoring network and automated water quality monitoring probes installed. Â The initial glance at that data didn’t show particularly problematic levels of oxygen either. Â So while we still need to look into oxygen levels, the current candidate is high temperatures (possibly interacting with lowered oxygen levels). Â Both of these kills began in the wake of very warm air temperatures. Â In particular, we had very warm nights preceding the onset of these fish kills.
A Brief Background on DO
Dissolved Oxygen
Water in our local streams, estuaries, and ocean isn’t just “water” but rather H2O plus a bunch of other stuff.  One of those key “other” components is a mix of gasses, essentially existing as very tiny bubbles throughout the matrix that is the surrounding liquid H2O.  We refer to such gasses as “dissolved.”  Dissolved oxygen is what most of the macroscopic critters you and I see in those water bodies breathe and utilize in their physiology.  As with any other constituent in our coastal water bodies, oxygen levels can fluctuate.  This fluctuation turns out to be a key driver of the types and diversity of organisms that reside in that water body.
Dissolved oxygen (DO) levels can vary significantly from place to place, over the course of 24 hours and over the seasons.  DO is also strongly dependent upon temperature, with other strong influencers including water depth/stratification, water movement, and nutrient concentrations.  In almost any coastal water body (but particularly the shallow lagoons and seasonal creeks common here in coastal southern California) during the hot, still days of late summer and early autumn, fluctuations in all these factors can create locally low DO levels. That DO can get to the point where oxygen is effectively depleted from water as far as our fish and related organisms are concerned.  If we hit such low DO levels, our resident fish populations have essentially two choices: move to a different location or die.  If the oxygen depletion is strong enough for a long enough time this will produce so-called fish kills or mass mortality events.  Generally, extremely low DO levels or massive swings in DO levels at any one particular site can be difficult or impossible to predict.  But eutrophication, stagnation (e.g. no wind to promote mixing nor running water), and high heat are frequent triggers here in southern California.
Blooms and Oxygen
Like all photosynthetic organisms, algae and aquatic plants produce and release oxygen during the day and take up oxygen during the night (in the so-called “dark cycle” phase of their daily photosynthetic routine).  In our coastal waterbodies, this is often a major source of oxygen with benthic macroalgae and floating phytoplankton proving the lion share of oxygen to the water body.
As with any biological population, our populations of benthic and planktonic algae fluctuate.  Normally one would be hard pressed to notice these population changes.  But by late summer, our conditions here in coastal southern California often make such dynamics obvious.  We frequently see large carpets of Ulva/Enteromorpha (they form a species complex…and that will take more than this blog post to fully explain) blanketing our tidal creeks and mudflats or filling the emerald- and rust-tinted waters of our lagoons and ponds to signal a large algal population.  If this population explodes over a short period of time, we refer to this as an “algal bloom.”  Once that population booms or blooms, it typically busts in short order. A dense algal bloom produces a surplus of oxygen by late afternoon, that oxygen saturates the water column, oxygen begins to leave solution, and escapes into the surrounding atmosphere.  During the night (say around midnight to 3 am), that large algal population has a very high demand for DO.  The demand can be so high that DO approaches zero.  As large portions of that bloom begin to die (literally suffocating) due to either self-shading or depletion of the nutrients they need to grow, microbial decomposition starts to kick in and further drive up the demand for oxygen.  When this is severe enough we see very low DO levels for hours on end and (potentially) fish kills.
Algal Blooms and Fish Kills. Source: Western Australia’s Department of Water and Environmental Regulation.
Temperature and Oxygen
As water temperature increases, the capacity of water to hold DO decreases.  Holding all other factors constant and raising the temperature of the water will lower the DO in a water sample as the heat energy makes it harder for those water molecules to “hold onto” the oxygen. Water temperature also influences the rate of photosynthesis, the metabolic rates of virtually all multicellular aquatic organisms, and the sensitivity of most organisms to toxins.
As the most responsive component of water, temperature is very often a compounding factor. Â Something not always lethal or beneficial in its own right, but which has a strong magnifying effect. Â Exposure to a toxin when you are cold might do little or make you slightly ill. Â Exposure to a toxin where you are extremely hot might might make you severely ill or even kill you.
A New Normal?
So what are we left with as of this morning? Â The fish kill in Malibu seems to be mirroring a similar die off at Ormond Beach. While the sampling and laboratory analyses are still on going, as of this writing, we have no evidence of acute poisoning and a strong suspicion that these die offs may be being triggered by a larger scale environmental stressor. Â While the current, massive die off of marine life in Florida is different in scale and in proximate driver(s) than our unfolding story here in southern California, all these events may share a potential ultimate driver: that of sustained, large-scale transformation and degradation of our wider coastal zone. Â Recall that California has lost 91% of our wetlands over the past 150 years (50 to 95% if we are talking about wetlands in our various coastal counties).
These mass mortality events here in coastal SoCal may be the harbinger or a much warmer, degraded coastal system that is the harbinger of the new normal.  The Blob may be in retreat but the fact our climatologists and oceanographers are finding a need to invent new terms like “the blob” to describe persistent, modified environmental conditions is concerning.  So too is the fact Scripps Pier in San Diego recorded the highest temperature ever in its more than a century long record of daily temperatures. And our eroding coastline, spurred on by anthropogenic sea level rise.  More fish kills, less fish to eat, and a generally more fragile and stressed coastal ecosystem have long been among the obvious predictions of a changed climate. To be sure, we must wait some time to confirm these trends/explanations.  But the concern is that we have already crossed the Rubicon and that the increasing rate and magnitude of environmental degradation is something that needs to be reversed ASAP.
Recent Press Coverage
KVTA Morning Show, September 10, 2018
Ventura County Star, August 31, 2018
This VC Star story is behind a fire wall; excerpts are below for those without a subscription:
Scientists studying mystery of dead fish:Â Ormond Beach incident preceded one in Malibu
Cheri Carlson, Ventura County Star USA TODAY NETWORK
A few weeks before dead fish started showing up in Malibu Lagoon, hundreds were found dead at Ormond Beach in Oxnard.
Researchers from CSU Channel Islands worked with The Nature Conservancy, the California Department of Fish and Wildlife and others to try to figure out what caused the local dieoff, which started in late July.
Not the first fish kill
It’s not the first time a fish die-off has shown up in the area. In recent years, several small incidents have happened, some tied to pesticide spills, said professor Sean Anderson.
This time, groups got there quickly and collected fish. Necropsies were performed in the Camarillo university’s lab with help from a state fish pathologist.
Samples were sent to the state Wildlife Investigations Lab for testing. On Tuesday, as news spread about another die-off miles down the coast in Malibu, the results came back.
They were negative. “That rules out a chemical toxic exposure,” Anderson said.
Chemical exposure ruled out
That means something else was to blame, likely physical stress. And, it could be a similar cause of the die-off at Malibu Lagoon, too.
Right now, Anderson and others are leaning toward heat and low oxygen levels as the likely culprits.
An algal bloom could lead to low oxygen levels, but they saw no evidence of that happening near Ormond.
But the weather had been hot.
“Oxygen is super dependent on the temperature of water,” Anderson said. Colder water is called oxygen rich. Warmer water is oxygen poor.
At Ormond, most of the dead fish that initially showed up are long gone.
But there is still a lot of data to study, Anderson said.
Finding a cause for dead fish
He and his students plan to dig into atmospheric records to get more details about whether it really was “exceptionally warm” during the lead-up to finding the dead fish. He also would like to put some sensors in the water.
First up is to look at toxicology data from the Malibu fish to confirm the die-off also wasn’t the result of some kind of toxic exposure.
Dead fish started showing up floating in the lagoon last week. Officials have said that came after several consecutive days of warmer-than-usual temperatures.
KVTA 1590 News Radio, August 29, 2018 (no audio at the following link)
KEYT 5 O’Clock News, August 29, 2018
