In 1997 I appeared before the California Senate Committee on Natural Resources (Chaired by Sen. Tom Hayden) to give testimony regarding emergency legislation to close the commercial Red Abalone fisheries in California, proposed by Sen. Thompson.
The legislation was largely a response to a study by K.A. Karpov and P. Haaker, two biologists employed by the California Department of Fish and Game. The report was presented to an ad hoc committee on Abalone on January 25, 1997. Karpov and Haaker painted a dismal picture of the Red Abalone population throughout most of California, except for a region off the north coast that is open only to sport divers, not commercial divers. There has been an ongoing phase-out of commercial abalone diving by the California Department of Fish and Game (DFG). First, they stopped issuing new permits for abalone divers. Then, they required that for a diver to acquire a permit, he or she had to buy two permits from currently licensed divers, and give one back to DFG. This reduced the number of divers by one every time two retired or changed occupations. Most recently, DFG prohibited any transfer of permits, so as divers retire or change occupations, no new divers replace them.
There had been other limitations on commercial abalone fishing introduced to protect the stocks. For example, the abalone season had been shortened several times, most recently in 1990, when the season became 9 months. Also in 1990, a daily "bag limit" on catch was introduced: 7 dozen in northern California, and 15 dozen in southern California. In spite of these measures—or perhaps because of them—the annual landings have generally decreased with time.
The two principal sources of evidence that the Red Abalone fisheries were in trouble are:
Let's look at these two pieces of evidence in turn.
Here is a plot of the commercial landings (annual catch in pounds):
There has clearly been a substantial decline in landings over the two decades 1975-1994. According to Karpov and Haaker, and M. Tegner, a marine biologist from the Scripps Institution of Oceanography, this decline indicates that the resource has been over-fished: that the commercial abalone divers have essentially destroyed the resource.
Do you find this a compelling argument?
What other factors might influence the total landings?
One factor that you might reasonably think would have a large influence on the annual catch is the number of abalone divers. If the number of divers decreases by a factor of four, you would expect the annual catch to decrease by a factor of four, ceteris paribus. (Unless, for example, some new technology is introduced that makes the divers more efficient, or that lets them stay on the bottom longer—they are quite limited in their diving time by the need to avoid "the bends," which results from long periods of breathing compressed air without coming up gradually (decompressing) periodically to allow the gasses to excape from the flesh. There have been no such technological innovations.)
is a plot of the number of licensed commercial divers during the same two decades:
Here is a scatterplot of landings versus number of divers:
Note that the correlation coefficient is rather large: 0.84. The association between the number of divers and the annual landings is strong. It is even more revealing to look at landings and number of divers, on the same plot year by year, to see the time-trends:
Some of the features of these curves have clear explanations, for example, the large drop after the first two years. What happened was that the sea urchin market opened up, and many abalone divers started diving for sea urchins instead. In 1976, there were 397 abalone divers, in 1977 there were 272, and in 1978 there were only 198. Not too surprisingly, the annual catch of Red Abalone dropped. The number of commercial abalone divers this year (1996) is 101; because permits are not being issued and existing permits are not transferrable, there can never be more than 101 in the future, only fewer and fewer as divers retire.
One might reasonably think that if divers are allowed to fish for only 9 months each year instead of 10, annual landings would fall. To put years with different length seasons on an equal footing, we should adjust for the length of the season. Again, ceteris paribus, if the season were shortened by 10% one would expect catches to decline by 10%, because the number of abalone a diver can collect is limited by the amount of time he can spend on the bottom. The next plot tries to put the 20 years 1975-1994 on an equal footing, by weighting the landings by the number of divers and the length of the season. The totals figures are standardized to the number of divers and length of the season in 1975.
The adjustments were computed as follows:
\(\text{Season Length adjustment}\)
\(\text{(adjusted catch)} = \frac{\text{(raw catch)} \times \text{(length of 1975 season)}}{\text{(length of season)}}\)
\(\text{Number of Divers Adjustment}\)
\(\text{(adjusted catch)} = \frac{\text{(raw catch)}\times (\text{# divers in 1975)}}{\text{(# divers)}}\)
\(\text{Adjustment for Both}\)
\(\text{(adjusted catch)} = \text{(raw catch)}\times \frac{\text{(length of 1975 season)}}{\text{(length of season)}} \times \frac{\text{(# divers in 1975)}}{\text{(# divers)}}\)
Does this seem like the right thing to do? What assumptions do you need to make for these adjustments to be reasonable? Are those assumptions justified on the basis of the other information we have?
One measure of the health of the fishery is the "catch per unit effort" or "CPUE:" how many pounds of Red Abalone divers catch in a given period of time. The following plot shows the CPUE for the two decades 1975-1994.
This plot and the previous one, with the normalization by the number of divers and the length of the season, show that the catch per diver-year has fluctuated somewhat, but if anything shows a positive trend.
When the sea otters, which are a protected species, returned to central California, they effectively razed the central California Red Abalone fishery. The density of abalone in sea otter range dropped by a factor of about 8. One would expect that to reduce the annual landings substantially; this effect is not accounted for in the plots above.
I believe these considerations show that the decrease in annual landings can be accounted for completely by the decrease in the number of divers, and the shortening of the season.
There remains the question of causality: did the decreasing catch lead divers to change profession, or did the exodus of divers cause the decrease in the landings, or neither. We actually know two of the important causes of the decrease in the number of divers: the opening of a lucrative sea urchin market in about 1977, and the DFG policies first of ceasing to issue new permits and requiring a two-for-one exchange of permits for transfer, and then of eliminating transfers completely, so that once a diver retires or changes profession, his or her permit is gone forever.
Recruitment is the word marine biologists use to mean the number of new animals in a population each year. Red Abalone recruitment has been measured in a number of ways, including
ARMs are the primary instrument used to measure recruitment in southern California. Most of the "invasive" diver surveys are in a part of northern California where everyone agrees the Red Abalone population is thriving.
Which of these survey methods would you think is likely to show the highest recruitment? If recruitment were the same in northern and southern California, where would you expect the measured recruitment to be higher?
If you want to compare measurements in different places made using different techniques, what should you do?
It is known that abalone are very sensitive to bottom conditions, and that they "don't like" cement, because of its pH and possibly its color. What would that do to measurements of recruitment made using cement ARMs?
Although individual abalone do not move far, abalone are "broadcast" propagators: their sperm and eggs mix in the sea and can be carried by currents some distance from the parents. When bottom conditions change, as a result of ocean currents, large-scale ocean circulation changes such as "El Niņo," pollution, or other factors, there tends to be a net migration of the population.
Suppose one started by making measurements in a few places where Red Abalone population densities were high. After a few years of ocean changes, would you expect measurements at those same places to remain high, if the overall population were not changing size?
We have been studying sampling for the last few weeks. How should the locations of the dives be chosen to get an unbiased estimate of the Red Abalone population? Of the Red Abalone population in regions that are fished commercially?
In fact, relatively few of the survey dives are in regions fished commercially. One reason is that much of the commercial Red Abalone is taken in areas infested by white sharks. DFG divers are apparently reluctant to go down in those areas.
In a recent survey of about 3200 Red Abalone (February 1997, a cruise off San Miguel Island), less than 2% were found to be of commercially legal size (7 3/4" maximum diameter and larger). A 1993 survey by DFG showed less than 3% of Red Abalone at the Fitzgerald Marine Reserve to be commercially legal size. Abalone begin to spawn at about 1 1/2" in size, and continue for many years. They spawn for 6-9 months a year. However, the viability of their spawn seems to decrease as the abalone get older: younger abalone are more effective spawners. Suppose, purely for the sake of argument, that
What must the recruitment rate be for this state of affairs to be stable, that is, so that the overall population does not decline?
Is the answer just that each abalone must have 2 viable offspring per year, or is it more complicated than that?
Suppose every Red Abalone had two viable offspring each year. What would happen to the population?
In the first year, there would be about 3 times as many Red Abalone, less the number that are caught by divers (about 2%) and the number that die of other causes (about 5%). This is clearly not stable—the population will grow very quickly. We need to think a bit more mathematically to answer the question.
The assumption that the mortality rate is the same for Red Abalone of all sizes (once thay are established on the bottom) gives rise to an exponential distribution of lifetimes. The exponential distribution is the analog of the geometric distribution for continuous time. For a mortality rate of 5%/year, and a growth rate of 0.7"/year, one would expect the fraction of commercially legal Red Abalone to be 57%, if there were no fishing, but we are assuming that the divers are perfectly efficient in gathering the legal Red Abalone 8" and larger in size.
The fraction of Red Abalone between 6 3/4" and 7 3/4" would be predicted to be about 10% of the total population. The fraction of Red Abalone between 7 3/4" and 8" would be predicted to be about 2% of the total population (roughly what's observed in some surveys).
The fraction between 1 1/2" (the approximate size at which abalone begin to spawn) and 7 3/4" (when they begin to be caught by commercial divers) would be predicted to be about 76%. Abalone of all sizes contribute to the mortality in proportion to their numbers, but only Red Abalone bigger than 1 1/2" contribute to recruitment. Under the assumptions we have made, this yields the conclusion that the recruitment required for stability is about 5%/76% = 7%/year.
Under the same assumptions, except pretending that there were no commercial abalone divers, what recruitment rate would be needed for the population to be stable? The answer is about 6% per year. The influence of the divers on the recruitment needed is an extra 1%, which is 17% of the recruitment needed for a stable population in the absence of divers.
This is only a model, and many of the assumptions are false (but not necessarily bad approximations to the truth), but it gives the flavor of the kind of computation one needs to make to understand the influence of harvest on population stability, which one might consider in balancing the possible benefits of closing the fisheries on the Red Abalone population against the certain deleterious effect on the divers and their families.
The "fishery-independent" measure of adult Red Abalone population is based primarily on emergent diver surveys. To the best of my knowledge, these surveys are carried out in more or less the same places in the years they are done, and the locations were not originally picked at random. In particular, because of white sharks, some of the most productive commercial Red Abalone fishing areas were not included in the emergent diver surveys.
If the original surveys were in areas of particularly high Red Abalone concentration, what would you expect to see as you revisit the sites, assuming that the Red Abalone population is stable?
From such surveys, what can someone conclude about the general adult Red Abalone population, including places not visited by the survey divers?
In a 1993 survey, about 26 sites were visited. What is your estimate of the standard error of the sample percentage based on 26 samples, assuming that the sites were chosen randomly?
SE = \(\dfrac{50\%}{\sqrt{26}} = 10\%\).
(26 data is too few to justify the bootstrap estimate of the SE, in my opinion.) If the sites were not chosen randomly, would this SE mean anything?
Suppose that DFG were estimating recruitment from these 26 dives, and pretend for the moment that (1) they represent a random sample from the potential Red Abalone habitat, and (2) the argument presented above about the recruitment needed to have a stable population is valid. Would it be possible to tell whether the recruitment is below the 7% threshold needed for stability (in our model) from a sample of size 26?
Given that the fishery-independent survey sites were not random and did not include some of the most productive commercial fishery sites, which measure would be a better indication of the overall status of the Red Abalone population, the CPUE, or the survey data?
On the basis of these data, do you think the legislation should be passed? If you were a legislator, how would you vote? Would you allocate funds for further study? How would you trade off the certain interests of the commercial divers against the possible but uncertain benefits to the environment? Keep in mind that extinction of Red Abalone is not the issue, because the population in northern California is doing well; the issue is a possible decline in the range of Red Abalone habitat.
By the way, the Senate Committee members present voted unanimously to pass the bill on to the full legislature for a vote. The bill ultimately passed.