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Fish Nutrition

By Rob Toonen. Posted to The Breeders Registry and Reefkeepers emailing list, Monday 22nd March 1999.

Some of you may remember that I was raising a clutch of juvenile octopus spawned here last fall. In my search for information regarding the nutritional requirements of the young, I entered into a debate about whether or not guppies were a suitable food item for the juvenile octopus. Rather than reproducing the entire debate here, I thought I would post a synopsis of some of the major points about feeding freshwater fishes to marine predators as an FYI....

A buddy of mine is a fish parasitologist at the University of British Columbia who volunteers with a veterinarian (DVM, PhD in fish disease at Guelph) at a couple of public aquaria to do autopsies on dead fish. He was telling me that the single most common cause of death he's seen since he started to do the autopsies at public aquaria is "fatty liver disease." Although not really a disease, fatty liver is a serious condition in which the liver becomes enlarged, often to the point that it interferes with the other internal organs and is apparently the cause of death. The bottom line from a number of exchanges I have recently had is that the most common cause of fatty liver disease is a diet high in saturated fats, although biotin and/or choline deficiencies, toxemia and "unknown nonspecific causes" are also possibilities (they cited FISH MEDICINE, by Michael K. Stoskopf, DVM, Ph.D, 1993, Saunders). My buddy said that he also sees the same "fatty liver" disorder in a variety of marine fishes (such as groupers and lionfishes) from pet shops where they were maintained on a diet of goldfish, and he felt the disorder was caused by the inability of marine animals to deal well with saturated fats common in freshwater fishes.

Aside from the fatty liver "disease," fatty acid deficiencies in fishes have been shown to result in reduced growth, higher percentages of muscle tissue water, liver degeneration, higher susceptibility to bacterial infection, and a decrease of hemoglobin in the blood cells among other nutritional problems (such as hair loss in mammals). Neither of my friends were sure of the exact fat profile of guppies or goldfishes, but both felt confident that freshwater fish in general (and feeder fishes in particular) were very different in fatty acid composition than are marine fishes. For fatty acids in which marine fishes are generally high, freshwater fishes are typically the converse.

They also thought that biotin was a possible concern for feeding any marine predator a diet entirely composed of feeder guppies or goldfish (biotin is a relatively expensive feed additive, and they suspected that commercially produced feeder guppies are likely to be biotin-deficient). However, freshwater fishes may contain slightly higher amounts of the B vitamins in general, and it is also possible that feeding marine predators entirely on feeder guppies could O.D. them on biotin as well -- it depends on the biotin composition of feeder guppies and goldfish (which they didn't know).

Because there is no real data for the nutritional profiles of aquarium fishes, I did a survey of the aquaculture literature to find the nutritional composition of feeder fishes to compare. Of course, I couldn't find the composition of guppies and goldfish, so I did the best I could. A quick comparison of farmed catfish & carp to cod & snapper (the most reasonable proxies for which I could find the exact nutritional composition in my search) and the values I could locate are included below. I doubt that the biotin issue is likely as important as some of the other nutritional differences between the freshwater and marine fishes (at least the total B complex values are not really different, but the saturated and short-chain fat values are *very* different between the two groups):

- Saturated Fat 18:2 18:3 20:5 22:6 B Complex
Catfish 1.77% 0.88% 0.09% 0.07% 0.21% ~3.5mg/100g
Carp 1.08% 0.52% 0.27% 0.24% 0.11% ~2.8mg/100g
Cod 0.08% 0.005% 0.001% 0.08% 0.13% ~2.5mg/100g
Snapper 0.28% 0.02% 0.004% 0.05% 0.26% ~1.5mg/100g

My friends generally felt that it was best to avoid feeding freshwater feeders to marine fish and vice versa because of the different fatty acids profiles of the prey items, and both thought that the primary secret to maintaining healthy fish/inverts long-term in aquaria had more to do with a varied diet than any specific nutritional component or additive. Given the limited data I could find, it looks like the nutritional profiles of freshwater fishes differs significantly (at least in terms of the saturated and short-chain fats) than that of marine fishes. Together with their experience from public aquarium fish autopsies, it would appear prudent to avoid feeding primarily freshwater fishes to marine predators in general. The primary recommendation from these sort of discussions (again and again) seems to be that aquarists need to simulate the natural diet and vary it as much as possible.

Very interesting info regarding the feeding of freshwater fish to marine fish. The concept that it is unhealthy is something I've been aware of for quite some time (and I think I had mentioned this to you before in a private email), although I really wasn't sure exactly how the excess fat was harmful.

It's something that we've talked about, and many people say off the cuff, but I have never seen any real explanation for it other than "it's bad for them." During a discussion with the folks on the Cephalopod list about raising my juvenile octopus, someone suggested using feeder guppies as a food, and I replied that I didn't want to use them because they were not a very nutritious food for a marine predator. They replied something to the effect of "prove it," so aside from contacting my Vet friends, I started to feed some of the isolated juveniles on feeders, and found that they grew more slowly and experienced higher losses than siblings in the same tank raised on other foods (such as live marine snails and crabs). Lots of us have had the gut feeling that feeding freshwater fishes to a marine predator is a bad idea for some time, but that post outlines the mechanism of how and why it is bad.

You have explained it very well. The only thing I didn't understand was the table. Was the explanation in the main post and I just missed it?

Umm, can't remember -- I sent it from work, and never received that digest for some reason, so I can't remember whether I simply didn't explain it. Sorry, I probably didn't explain it very well...

What were the headings? Were they some sort of ratio of saturated fat to unsaturated?

The headings were percentages of the total weight in a 100g sample of muscle tissue (a small fillet). They were compiled by the US government for nutritional comparisons of foods that are available to consumers. The lists are very detailed and have total nutritional content (everything from vitamins to trace minerals and gross composition). Unfortunately, there is little interest (or money) to develop similar data for aquarium fishes, so that was the best proxy I could find to get the actual numbers.

Thanks for the information. Any thoughts or ideas on using Sailfin Mollies?

It's funny, I've gotten several messages in reply asking if salt water acclimated guppies or mollies will be OK, and the honest answer has to be "I don't have a clue." The fatty acid profiles for those animals just aren't available and I am guessing as much as anyone else when it comes to this.

My gut feeling is that because other andromedous fishes (such as salmon and striped bass) are closer to the values for freshwater than they are for saltwater, but are somewhat intermediate, guppies and mollies would be the same... But like I said, I am really just guessing here -- I just can't find the fatty acid profiles for any of the aquarium species.

I wonder how the fatty makeup compares to "strictly" marine species? These fish are now many generations removed from a FW environment.

David Cripe and I were just talking about this when I visited him last week, and there may be some hope that we'll see some actual data on one such case somewhere down the road. Until someone actually analyzes the fatty acid profile of freshwater and saltwater-acclimated guppies, though, we have no real idea how "good" or "bad" those feeders really are.

Given the (admittedly) limited data available, I think it just a reasonable precaution to avoid using such feeders as a sole or primary food item for extended periods of time. I doubt that the occasional guppy or molly is likely to cause any longterm problems (sorta like the occasional trip to Jack-in-the-Box ), and David is using them successfully in his rearing program, but if you feed nothing else for extended periods of time, your fish are probably destined for the same fate as you if you were to eat at one of the burger chains for the rest of your (shortened) life...

At least that's my guess given the lack of data we have on the guppies and mollies to date.

Jake Levi

Ref to saltwater guppies, aint no such animal. I have long tried to acclimate them, it doesnt work. I have gotten females to half strength in good health, the males dont last. But the females will have fry that last longer then total freshwater fry in salt water. Other species like gambusia acclimate better. Mollies do the best, but, even though they acclimate to full marine the fry dont do as well as in freshwater. I have found tht its best to keep mollies in half strength marine water, get larger broods that way and when put in salt water the fry live. Guppies are a disappointment unless you keep them in half strength marine level salt. gambusia are a better choice. Even with them many males dont last as long.

What the fatty acid values are I dont have a clue, but, I also think that one or two a week wouldn't do harm.

Stanley Brown

I don't know what is a typical number of young per birthing for animals in FW but the larger females routinely produce 30 - 40 young. Many do get eaten as I make no attempt to sepparate the young, but they seem to be quite prolific. Perhaps this is due to the 8+ years that I've had this population established. I started with the green sailfin and the mottled b/w variety; 1 male and 2 females of each. The green sailfin were the more prolific and the population is now almost exclusively this coloring, what remains of the mottled coloring could be "throwbacks".

At one time I had them breeding in a tank of greenwater , the algae density such that you could only see the fish if the were at the surface or next to the glass.

Jake Levi

I think that you have a definite 'evolved' and selected- for population. Linda Barnes of the former seabase hatchery in Utah has a population of mollies in the spring there also, but, by and large, it takes quite a number of genertions to select for high survival in full sea water.

Just my opinion based on a few years of doing it. My present population is a dozen or so generations and the fry broods are seldom 30, more on the average of a dozen to twenty while a parallel population in FW has over 40 per brood. Definite difference even though from same stock.

David Cripe

I have had guppies accimate from fresh to saltwater. As Rob said, I use guppies for a very short (5-10 days) period in rearing scombrids (tuna). Some of the guppies that are put in as feed, find a refuge in the cage that surrounds the heater and survive for extended periods of time (until they venture out and get eaten). In addition, there is a commerical hatchery that is currently working on producing "saltwater" guppies in both small (10 mg) size and larger.

Created by liquid
Last modified 2006-11-24 22:42