Brackish Water Fishes

 by Bill Cline

A topic as broad as "Brackish Water Fishes" can't be covered well enough in a single field trip or lecture or even the two together.  I'm going to give up on a discussion of the individual species, except as examples.  

What Is Brackish Water?

Ideally, freshwater is pure H2O. You would never find pure water....even a glass container dissolves a tiny bit. Saltwater has the salinity of the oceans  but that's even harder to define. Every sea has a different amount of salt in it, and the chemicals making up that salt vary too. Aquarists don't really need to worry about these details, they just help in understanding the following 'confusing' things. We either mix up our brackish water as a percentage of seawater, or measure the salinity of the 'mixed up' seawater with a hydrometer or some other device. When they first put the metric system of weights and measures together, they decided that a "gram" would equal the weight of a cubic centimeter of pure water. A hydrometer then floats at the 1.000 reading, meaning that the weight of the water it has displaced weighs exactly one gram. All the different salts in a cubic centimeter of average seawater weigh .025 grams, so it pushes the hydrometer up till it reads 1.025 (the weight of the displaced water, plus the salt dissolved in it). I always thought "brackish water" could be anything in between, but apparently there are rules I wasn't aware of. Freshwater is considered to be 1.002 or less, and saltwater is 1.018 or above. Brackish water is therefore anything between1.002 and 1.018 grams per cubic centimeter.

You think you're confused now? Some books give you a fish's required salinity in percentages  like 60% seawater. That's pretty easy to understand; to fill up a 10 gallon tank with 60% seawater, you'd put in 6 gallons of seawater and 4 gallons of freshwater. But how do you know if you can keep two species together, if one book says the first species should never be kept at less than 70% seawater, and a different book says the other fish should never be kept at anything over 1.019? Well, it sounds worse than it is. What you need to do is figure out what 70% of the weight of the salt in a cubic centimeter of full strength sea water is. If you multiply 0.7 (from 70%) times .025 (the salt in that much pure seawater), you get about .019. So the fish can be kept together at 1.019 (but you'd better be careful the salinity doesn't change more than a little). A few odds and ends about brackish water might include: 1) Because salt corrodes metal, and metal can be poisonous to fish, don't use metals with a brackish water tank. And 2) It's better to dissolve the salt in water before adding it to the aquarium, rather than dropping the crystals directly into the tank. It might take considerable time for the salt to mix with the water, making the hydrometer readings kinda unreliable.  

What Is Brackish Water? 

Beyond the hydrometer readings I already discussed, I was curious about which chemicals might be required to make it "brackish". I looked in a couple of books, but they both referred to diluted "seawater", so I guess the Great Lakes of Africa are not brackish, no matter what the hydrometer said. However, the Caspian Sea, Aral Sea, and parts of the Baltic and Black Seas then must be. On the other hand, the books I used for my discussion of hydrometer readings for "fresh", "salt", and "brackish water earlier, made no restriction as to the source of the salt. Perhaps I'm just splitting hairs, er..........scales.

A Rose Is A Rose, But A Saltwater Fish Isn’t A Freshwater Fish

Osmosis refers to the tendency of chemicals to move across a semipermeable membrane, from areas of greater concentration to areas of lesser concentration.  To put this in ‘aquarist’s terminology’, if a guppy is in a breeding trap, the freshwater in the trap is calmly in balance with the water on the other side of the trap - just as much water flows into the trap as flows out.  When the guppy gives birth, the babies are much more concentrated inside the trap than they are outside it.  So they spread into the water outside the trap till there are about the same number per cup of water outside as there are inside.  When a baby crosses from inside to outside, just as much volume of water flows back inside to replace it.  For similar reasons, a freshwater fish in freshwater tends to lose salts from their bodies into the water, and absorb water molecules from the water around them.  That’s because their blood has more salt than the freshwater around them.  Any of you who have seen fish with the condition which used to be called ‘dropsy’, causing their body to soak up water like a water balloon, are familiar with what would happen to a freshwater fish if it wasn’t able to offset this osmosis.  Fortunately, they are able to.  For one thing, to avoid soaking up water, they don’t drink any.  This is why, if a fish has disease ‘bugs’ inside their bodies (under the skin), you can’t treat it by putting medicine in the water.  Since they don’t swallow water, you have to get them to swallow medicated food.  When they swallow food, they move it back in their mouths till it’s packed in front of their esophagus.  Then they swallow it with as little water as possible.  A fish’s blood cells don’t flow out of the body, because they are like the mother guppy..... too big to fit through the holes/pores.  A little water still seeps into their blood, so to get rid of this excess water, freshwater fish urinate a lot, but with very little body salt in it.

On the other hand, saltwater fish have the opposite problem.  Water is being drawn out of their bodies, into the surrounding seawater, because it’s saltier than the fish’s blood and cells.  Bony fishes (just about everything but sharks and rays), resist that by drinking lots of water, and getting rid of the excess salts through special salt secreting cells in their gills and urinary system.  The cells in the gills get rid of small salt ions, like sodium and chlorine.  The cells in the urinary system get rid of larger ions, like carbonate.  Oddly, sharks and rays resist it by putting more urea into their blood, till the concentration of water is just as strong in their blood as it is in the saltwater.  It’s also why shark meat tastes somewhat different from the meat of bony fishes.

You don’t normally find families of fish, adapted to salt or fresh, with more than a few species adapted to the other kind of water.

Because of the different ‘equipment’ required to live in each kind of water, you very seldom find fish able to live in full strength fresh as well as full strength salt water (euryhaline fishes).  Salmon and eels are well known for this ability.  Most other euryhaline fish, while able to live in the extremes, are much healthier at a salinity in between the extremes.  Aquarium fish such as these include the flagfish and its lesser known relative, Jordanella pulchra, the many species of North American Cyprinodon and Fundulus killies, the Aspredo banjo catfish, the Columbian shark catfish, the estuarine Mystus catfish (M. Gulio), four-eyed fish, pike livebearer, mosquitofish and many kinds of mollies, mudskippers, bumblebee gobies and many other gobies, peacock blennies, chromide cichlids, jewel cichlids, what used to be called Tilapias, freshwater sole, Datnioides, halfbeaks, needlenose, sticklebacks (G. Aculeatus), monos, scats, most archers, glassfish, and targetfish (Therapon jarbua).  I’ve abbreviated the list for the sake of space, but if you want the particulars, check the book Neale Monks edited, Brackish Water Fishes (2006).

Most of us have heard of species which live their lives in the ocean, then migrate up rivers to spawn (anadromy), and then usually die.  Salmon are the most familiar, but sea catfish (Ariidae) and sculpins (Cottidae) are the only aquarium fish which also breed this way.  On the other hand, there are other fish which grow up in freshwater, but migrate to the sea to breed (catadromy).  True eels (Anguillidae) and mullets (Mugilidae) do this, but about the only aquarium fish which breed this way are some flounders (Pleuronectidae) and basses (Moronidae).

There is an even smaller group of fishes, which stop to breed when they get to brackish water, rather than going all the way.  Among the aquarium fish which do this are some snappers, groupers, jacks, and batfish.   

What Is An Estuary?

My Oceanography textbook (1970) by Peter K. Weyl and California Water (1971) ed. by David Seckler both define an estuary as, "...a semi enclosed body of water that is connected with the ocean, whose water is measurably diluted by freshwater derived from the land". There seems to be more involved in the definition though, because I've read that the definitions of "bay" and "estuary" only differ in 'how much freshwater flows into how much saltwater'. Intermediate bodies of water are very hard to put a name to. Oddly, in estuaries which are fairly calm, there is usually little vertical mixing between the lightweight freshwater on top and the heavier saltwater underneath. As a result, you frequently get pure freshwater flowing all the way out to the waves over pure seawater.

Why Does So Much Stuff Grow In An Estuary?

Plant and animal communities in estuaries have three things in common: 

1)Few species

2)Lots of productivity

3)Adaptability to change. 

An adaptability to change would be obvious. With the salinity changing at times from saltwater to freshwater, with the temperature fluctuations of tidal channels which are often no more than 6 inches deep and 2 feet across, with heavy doses of pollutants coming through at times, life has to be hard. With life so hard, few species would be able to adapt. The few which are able to adapt are able to produce huge populations, with there being a huge food supply. Most of this is not too hard to understand, except for the 'huge food supply' part.

Lots of productivity essentially means a huge food supply. The productivity that a farmer would think of is simply, "How much produce can I get out of this land per acre in a season?" To a scientist, however, it means something much more complicated. It means, uhh.......... well, pretty much the same thing, but how do you measure it? Just as the farmer would be mainly interested in how much vegetables per acre his land produces, an ichthyologist would be most interested in the pounds of fish the river or lake puts out. In our case, however, we would have to deduct the weight of the starting fish population, whereas the farmer didn't have a starting population. If we were in his shoes (only interested in the productivity of the plants (algae, vallisneria, water lilies, etc)), we would call that the "primary productivity", primary referring to plants as the base of all the productivity. Every farmer though has to admit that some of his crop gets turned into rabbit food, snail food, aphid food, etc. It also raises a bunch of birds, snakes, and lizards, which have little effect on his crop, but which are being 'produced' on his land. He would also have to include weeds. Along the same line, aquatic biologists frequently study the productivity of the whole ecotype, from bacteria and algae all the way up through fish eating birds and bears. It can become a WHOLE lot of work.

Just as there are several kinds of productivity to study, there are several different ways to calculate it. If one was studying the productivity of the California killifish in the Tijuana Estuary, it would be necessary to calculate or count the population of that fish in a representative part of the estuary, contain them, protect them from predators, then calculate or count the population at the end of the period. If we subtracted the beginning pounds from the ending pounds, we'd know how many pounds of killifish were produced in that period of time by that number of square feet of estuary.

Trying to calculate the productivity of the whole ecosystem requires some assumptions. Rather than knowing the starting weight of every species of bacteria, algae, vascular plant, invertebrate, fish, bird, reptile, salamander and mammal, they usually calculate it, using the rule that the production of the whole ecosystem will be based on the production of the plants. When plants photosynthesize, 1) oxygen is produced, 2) CO2 is used up, and 3) carbon is taken from CO2 and converted to organic matter. All three have been used to measure primary productivity (productivity of plants), and to make estimates of total productivity of ecosystems. You may wonder why I'm spending all this ink on productivity. It's because estuaries have some of the highest productivities on the planet.  

 How Much Productivity Does The Typical Estuary Have?

Well, one of my ecology textbooks,  Readings in Population and Community Ecology, (1970, by William E. Hazen) has an article by John H. Ryther with a table (pg. 312) which gives the highest productivity to a turtle grass flat (11.3 grams dry weight per square meter per day). It was followed by a coral reef (9.6 grams), a polluted estuary (8.0 grams), the Grand Banks (6.5 grams in April), the ocean over the continental shelf (0.4 grams), and the Sargasso Sea (0.4 grams), to name a few. The highest ecotype on land was a sugar cane field (18.4 grams), but that was for a whole growing season. The lowest on land for a growing season was a desert (0.2 grams), in between for a whole season are a tall prairie (3.0 grams), a pine forest (6.0 grams), a field of wheat (4.6 grams), a field of rice (9.1 grams), a Spartina marsh (9.0 grams), and a short prairie (0.5 grams). As you can see, productivity of estuaries is among the highest in the world. I seem to recall reading that the productivity of an estuary was about the same as that of a tropical rain forest (but I can't remember where I read it).

Why do you suppose an estuary would have such high productivity? That's because of the differences between freshwater and saltwater. Freshwater flows down the rivers carrying silt, clay, nutrients, pollutants, iron, aluminum, nitrogen, phosphorus, silicon. Seawater has calcium, magnesium, sodium, potassium, chlorine, and sulphates. When they mix together in the estuary, many of them react with the other chemicals, forming chemicals which are no longer soluble in salt water. So they form particles and sink to the bottom. Saltwater has a different pH (8.0 to 8.2) as opposed to the more acidic readings for freshwater. This causes clay and silt to form larger particles and sink to the bottom. Before they get there, though, zinc and other metals attach to them. As a result, deltas are formed out of tiny mud particles at the mouth of most rivers, and they carry with them all kinds of nutrients. This is called "flocculation", and as soon as it hits the saltwater, it causes algae, vascular plants, invertebrates.....essentially everything to bloom.

 Not only the life that lives in the estuary, but bigger food fishes from the ocean move up there to breed, so their babies will have more food  or they breed out in the ocean, and the babies swim to the estuaries on their own. Among the food and game fishes, whose babies have been found in the Tijuana estuary are California Sheepshead, Pacific Mackerel, California Barracuda, Queenfish, California Corbina, White Croaker, Barred Sandbass, Spotted Sandbass, Kelp Bass, Walleye Surfperch, Shiner Perch and California Halibut. This being an ecological preserve, of course, everything is protected. Aside from that, it is the FRY of those game fish that might be here, and you don't want to eat a fish only a couple of inches long.

Other fish which have been well kept in aquaria, and have been found here include California killifish, Longjaw mudsuckers, Yellowfin gobies, Arrow gobies, Cheekspot gobies, Bay gobies, Shadow gobies, Mosquitofish, Opaleye, Bay pipefish, Bay blenny, Scaleyhead sculpin, and Staghorn sculpin. Also because this is a preserve, if I find that you've been collecting here, I'll grind your bones to make my fishfood.

Some of you may have noticed that I mentioned the mosquitofish as an occasional member of this community. They may be the only member of the family of freshwater aquarium livebearers (Poecilidae), but they are far from the only livebearers to have been found in this estuary. I remember, as a young boy, when I caught my first surfperch on a rod at the beach. I remember it well, because when I got it up out of the water, baby fish started streaming out of its belly. Livebearers in the estuary have included the Barred and Walley's Surfperch (Amphistichus argenteus and Hyperprosopon argenteum), the shiner perch (Cymatogaster aggregata), the round stingray (Urolophus halleri), the batray (Myliobatus californicus), the Shovelnosed Guitarfish (Rhinobatus productus), to say nothing of the Bay Pipefish (Syngnathus leptorhynchus).  

A Garden?  In Brackish Water?

     There are several plants which will do all right in slightly brackish tanks (up to 1.003), but don’t naturally occur there.  That makes these plants suitable for any water in which freshwater fish, which are tolerant of salt might be put (tire-track, peacock, and fire spiny eels, reedfish (ropefish), the Pristella tetra, many goldfish and koi, suckermouth cats (Glyptoperichthys gibbiceps, Hypostomus punctatus, and Liposarcus pardalis), the Asian red-tailed cat, the channel cat, and the two Mystus species (M. Vittatus and armatus), guppies, swordtails, the variatus and maculatus platies, Aplocheilus killies, climbing perch, giant gourami, Asian snakehead, Nandus nandus, convict cichlids, blue acaras, firemouth cichlids, most puffers, and the Atherinids (Bedotia geayi, Telmatherina ladigesi, and most Melanotaenias)).  Some of these plants include; Anubias barteri var. nana, Ceratopteris cornuta, Crinum thaianum, Cryptocoryne wendtii, Hygrophila polysperma, H. corymbosa, Vallisneria gigantea, and V. asiatica.  Neale Monks, in his Brackish-Water Fishes (2006) includes Ceratophyllum demersum in this list, but describes the plants roots in some detail.  Because I have never seen hornwort grow roots, I’m unsure if he isn’t describing a different plant.  Later, he lists aquarium plants capable of living in salinities up to 1.005, but which do naturally occur there: Bacopa monnieri, Crinum calamistratum, C. pedunculatum, Echinodoras tenellus (chain sword), Lilaeopsis brasiliensis, and Microsorium pteropus (Java fern).  In the end of that chapter, he mentions a brackish-tolerant plant, which he says is well-adapted to life in an aquarium........ a mangrove.  My goodness, a mangrove is a tree.  Maybe it’s just my lack of experience in dealing with mail-order brackish-water plants, but I’ve never seen a mangrove growing in an aquarium.  I would love to though.  Maybe they just need to be kept cut back.   

Cycling a Brackish Water Aquarium

When cycling the bacteria in a brackish water tank, get starting cultures from the kind of aquaria closest to the salinity of the tank you’re aiming for.  If you’re going to have a low-salinity brackish water tank (less than 1.005 specific gravity), add gravel or filter cultures from a freshwater tank.  If you’re aiming for a high-salinity brackish water tank, use gravel or filter cultures from a saltwater tank.  The bacteria aren’t interchangeable.   

Odds and Ends About Brackish Water Fish

Unlike saltwater or freshwater fish, brackish water fish not only survive changes in salinity, whether it’s long-term (scats and monos spend their youth in mostly freshwater, migrating to much saltier water for adulthood) or short-term (heavy rains or very high tides).  These changes in salinity will frequently kill off parasites and disease organisms.

Active fishes which zip around are most likely to rapidly go through waters of widely variable salinities.  Territorial fishes, such as many of the gobies, encounter much less variability.

Tijuana River Estuary - A History of Disasters

I grew up in this community.  My family moved here in 1958, when I was 9 years old.  That is an age filled with adventures.  Of course, the beach was there, full of pretty girls in bathing suits.  But until women took over from childhood adventures, this estuary was at least as entertaining.  There were miles of informal bike paths, with which to visit rabbits, horny toads, snakes, lizards, burrowing owls, red-winged blackbirds, ponds, and lots of farm crops.  I used to walk through these lands munching on tomatoes, strawberries, and lots of sweet corn.  But the water was the most fascinating.  I had seen sand-dollars on the beach, but I never saw a live one till I followed the beach, through the mouth of the river into what we called the “T. J. Sloughs”.  When a sand-dollar was alive, I usually saw it in the water, half-buried in the sand, with its shell sticking out of the sand in a perpendicular fashion.  Not only that, it seemed to be covered in rough suede.  Finding a patch of them, they would be about 2 feet apart from each other.  Even more common were the nudibranchs.  They seemed to resemble slugs from the garden at home, except that their body was folded up into that shape.  The top was divided into halves, and if you unfolded it, it was actually shaped like a small pancake, about 4 inches across.  Not only that, the color was deep, deep black.  Against this dark background, the perimeter had a bright blue line, and across the body there were probably 40 gold-colored, tiny dashes.  I’d watch them till my terminally short child’s attention span caused my eyes to look for other things to investigate.  There was a time though, when I moved a sea snail into the path of one of these beautiful critters.  As I stooped there, not knowing what to expect, the nudibranch made contact, and ‘jumped about half an inch, in surprise.  It got over it’s surprise very quickly, and in an instant, sucked the snail inside totally, shell and all.  Another nudibranch to be discovered, though not as common, was the ‘sea hare’.  It’s color was nice, kind of dull yellowish gold, but what made it so interesting was its size.  I swear, the thing was 10 or 12 inches long and about 5 or 6 inches tall.  There was always the occasional splash from a fish jumping, or the screech of a hawk, but the adventures in the estuary always had so much meat to chew on (figuratively).  When I took high school biology, quite naturally, I signed up for a project in the estuary.  Having filled petrie dishes with various agar cultures, I took samples of water from different spots in the estuary and ‘planted’ them in the petrie dishes.  I didn’t take the project very far, but it was fun.

The problem was, San Diego just kept growing.  All up and down the coast, wetlands like this were being turned into shopping malls, gas stations, and apartment complexes.  People weren’t appreciating the true value.  In fact, I had no appreciation of its size till I started this project.  I’d always thought that the drainage for this river went about two miles, crossed into Mexico, and dried up on one of the hillsides in Tijuana, probably in our view.  The watershed for this river (448,323 hectares) crosses back into California in the east county, and includes Lower Otay, Barett and Morena Reservoirs and their watersheds, as well as down in Mexico as far as San Juan de Dios, and including Rodriguez Reservoir.  That is a huge watershed, but since it’s very dry land, it doesn’t get much water.  All the people here now make it unlikely that the dams will release much water.

Before 1978, a number of studies found 29 different species of fish in the estuary, 17 of which were adult or young food or game fish.  Two were bait fish, and 13 would have made nice aquarium fish (mosquitofish, California killifish, longjaw mudsuckers, 5 species of gobies, opaleye, bay pipefish, 2 sculpins and a blenny.  The dune vegetation was in decline, as was much of the overall ecology.

The growth of Imperial Beach from 1928 till 1978 was very quick.  In 1928, there were just over 50 dwellings, all within 3 blocks of the beach.  Ream Field Naval Air Base was opened on the eastern edge of the estuary.  It became the largest helicopter base in the world, and brought with it hundreds of families needing houses.  Now it’s much smaller and goes by the name, Imperial Beach Naval Base.  By 1953, Imperial Beach had built a sewage treatment plant there and gravel was being dredged out from just south of the sewage facility.  When my family moved here in 1958, it was mostly houses within 2 miles of the beach.  Houses and apartments had been built on the narrow stretch of beach and barrier sand dunes separating the ocean from the estuary.  That plus heavy recreational use of the dunes killed off most of the plants which were trying to grow there (they held the sand in place).  Without them, high surf, storms and floods occasionally washed over into the tidal channels.  The flooding in 1978 killed off the sand dollars, and they didn’t return till 1985.  There was major flooding again in 1980 (highest stream flow readings ever)  And again in 1983, when record setting El Nino storms washed much of the sand comprising the barrier dunes into the estuary.  This made the lagoon shallow.  With the shallow lagoon and the drastically reduced flow the next year of freshwater from the Tijuana River (people gotta water their lawns, ya know), the mouth of the river got closed by nature.  Sometime around April 8^th of 1984, the shortage of water flowing down the river, and the lack of a deep channel for the water to flow through, just ‘disconnected’ the estuary from the ocean.  After this happened, the U. S. Fish and Wildlife Service began a plan to dredge it out, but it took 8 months to clear all the permits (mid-December).  With nearly zero rainfall that year, the waters in the estuary became much saltier than seawater.  Much of the marsh vegetation began dying out, but it started to rebound in 1985, when the tidal flushing had been restored (though the percentages of each species had been changed forever).

Because the estuary here is mostly influenced by saltwater, with the freshwater having a minor influence, the above floods, when much of the saltwater got washed out, had a great effect on the biology here.  So did the period of hypersaline waters, when the mouth was blocked off.  The floods greatly reduced the number of fish species in the estuary.  The topsmelt became fewer and smaller, while the arrow gobies and California gobies became dominant.  Then while the mouth was closed off, topsmelt, California killifish, and longjaw mudsucker made up more than 99% of the fish sampled.