Fish Fecundity

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Off Canada's east coast, Fisheries and Oceans Canada (DFO) scientists have been examining egg production rates - often referred to as fish fecundity - in a number of fish species. Understanding and measuring fecundity is important because it is the starting point for recruitment (how many individuals come into a population). The recruitment and then growth and survival of those recruits are what determine how many fish you can take out of the population. The factors that affect growth, condition, maturation and fecundity (egg production rates) are all critical to determining the level of fishing a population can sustain and in determining the length of time it will take depleted populations to recover. The starting point of all this is finding out how many eggs are produced and that's what fecundity is all about.

Enter DFO scientist Joanne Morgan whose research focuses on factors affecting productivity in ground fish species. Along with her colleague Dr. Rick Rideout, she has been examining fecundity rates with three populations of cod – northern cod, southern Grand Banks cod and southern Newfoundland cod. In the same area they have been looking at flatfish species – three populations of American plaice, two populations of witch flounder, and one population of yellow tail flounder.

American Plaice eggs

Yellowtail Flounder eggs

These photos were taken at the same magnification, demonstrating the difference in size that can be found in the eggs of different species. Photo by: Dr. Joanne Morgan

The scientists have been looking at variation as it plays out over time and space. Within that context, there arise a number of questions. How much variation is there? Is there a factor that you can use to reliably predict how many eggs fish are going to produce? Notes Dr Morgan, "We look at the changes with population size and at length and weight and different measures of fish condition, all to try to see if we can come up with one factor that could reliably predict, from year to year, how many eggs will result. So far, we haven't been able to do that."

They have also been looking at how much fecundity variation influences a population's productivity. You have variable amounts of eggs being produced per individual female, but when you scale that up to the population as a whole and try to estimate how many eggs the entire population is producing, does it really make a difference or does it all just come out in the wash? In fact, if you incorporate this variability compared to just assuming that the fecundity rate is constant, it can make a very large difference in what your estimate of total egg production is.

An added complication is to understand which variables are the most significant to consider. For example, asked to comment on how base population size can affect fecundity, she replies, "There are two seemingly contradictory factors at work here. One is that if you have less fish, if your population is smaller, assuming each individual has the same fecundity over time as they've ever had, then you'll have less total egg production. But there have been some studies, which have indicated that if population goes down, you'll get a sort of compensation, and that individuals will have a higher fecundity rate. They'll produce more eggs per individual to somewhat compensate for the lower population size."

Complications and contradictory evidence are all part of the equation. They found some indication that there was an increase in fecundity when population abundance went down, but then when they followed the populations farther, one population increased and another didn't. But both showed a subsequent decline in fecundity and in the case of cod there was little evidence that populations in Newfoundland were able to compensate by increasing their fecundity when population size decreased.

Dr. Morgan: "We've found that fecundity was much more variable than we had originally thought, which makes for difficulties in estimating total egg production. The fact is we haven't yet been able to find what's causing the variability. Dr. Rideout and I are working on implementing new techniques that have been developed elsewhere that will make the actual counting of the eggs much, much quicker, so it should be much easier to monitor on an ongoing basis. Of course the more data we can collect the better we'll be able to understand what causes the changes that we see."

Close up of fish ovary. Photo by: Dr. Rick Rideout

The scientists are looking into the reasons for the variability they are witnessing - including for example, changes in weight, in length, and liver condition. They really haven't found anything that consistently has the same relationship from year to year with fecundity. Fish sampled in one location have been exposed to a very different set of circumstances than fish sampled just a few nautical miles away. So it stands to reason that there is a lot of variation because individuals have different histories and are exposed to different environmental conditions. What researchers like Dr. Morgan are hoping for is to discover some signal within that variation that will help explain or understand what it is that causes the differences that they see.

"In an ideal world", she notes, "our hope is that with more data, which we're working on getting now, that we will be able, not only to just monitor and see what's happening, but get a better handle on what's causing the changes. Most of the samples that we have worked with so far are parts of historic collections. They were already here and they needed to be processed. We were able, over the last few years, to have some very good people working with us to process the samples. More recently we have been looking at egg size using a new method called the auto-diametric method to measure fish fecundity. It is much faster than the older methods were. We hope that with more data we will be able to determine which factors are having a significant impact on fecundity."

So in the scheme of things, just how important is the matter of fecundity? Dr. Morgan: "We think it's very important. Obviously, you need to know how many fish there are and perhaps that's the most important aspect. But all fish are not created equal and there's a lot of variation in productivity. Fecundity is part of the productivity equation, so we think it's important to not only know what the fecundity rate is, but to understand what factors are playing a role in changing reproductive profiles."

It is important work that will continue. Answers to the fecundity riddle will provide important information that can be incorporated into appropriate scientific advice for future fisheries management.