Fishing about and about fishing
M. Ben-Yami Column October, 2014
SELECTIVE SCIENCE
Nine years ago, I wrote here a column entitled "IS SELECTIVITY WRONG?"
It told how since the dawn of modern fisheries releasing the smaller and younger fish from nets has been management’s dogma, and how we’ve been always told to fish only or mainly older, larger fish, and let the younger and smaller fish grow, mature and procreate. Hence, meshsize and hook size regulations for selective fishing. But, since the early 2000s, fisheries scientists have started question this old, almost axiomatic assumption. Most of the sceptics came to the conclusion that sustained creaming off the larger and more prolific individuals brings about dwarfing in exploited fish populations. An argument even started among them as to whether selective fishing also causes genetic evolution in fish stocks, or the process is reversible.
Blaming ocean's warming. But now, in a study, funded by Marine Scotland Science (MSS), published in Jan. Issue of Global Change Biology, Dr Alan Baudron, Research Fellow at the University of Aberdeen, and his co-authors are linking the decline in the length of fish in the North Sea to climate change. Dr. Baudron and his colleagues used 40 years of age and length data of commercial fish in the North Sea collected by the International Council for the Exploration of the Sea. Over a 38-year period, the maximum body length of haddock, whiting, herring, Norway pout, plaice and sole in the North Sea has decreased by as much as 29%, coinciding with an increase in water temperatures of between 1 and 2 deg. C.
Since these six species, differ in their biology, fishing mortality, eating different diets, living at different depths, the authors' reasoning is that main villain are the increasing water temperatures, however subtle (up to 2 deg.C).
According to Dr. Baudron, those findings are consistent with current understanding of the physiology of fish that are “cold-blooded” animals and their metabolic rates are determined by the ambient temperature. “In general, fish grow more rapidly during their early life when temperatures are warmer. The consequence of rapid juvenile growth is that they become mature at a smaller length and therefore don’t grow as large as they would have in colder waters".
But, wait a minute! He also said that ..."the synchronous reduction in length did not apply to all species – cod, for instance, did not conform to it. It is also important to remember that these other factors - food availability and fishing pressure - may have impacted growth individually for each species along with the common effect of temperature, and will continue to do so into the future". And Dr Coby Needle of Marine Scotland Science (MSS), Aberdeen, added: “This study contributes important information to help us address the issue of how fish growth is changing through time and highlights the importance of considering environmental issues when managing fisheries.”
Ignore selectivity – inertia must prevail. Somewhere between Scotland and ICES, of which Scotland must be a member, something got lost: the problem of selectivity. Without questioning the data in the Aberdeen study, one cannot agree with the authors' concluding reasoning. This, because decrease in the average size of individuals in the Atlantic cod population has been documented long time before any ocean ("global") warming was observed and reported. The earliest report comes from almost 100 years ago, when according to Hansen's 1949 paper, the average age at first maturity of Atlantic cod at West Greenland declined from 9.9 years in 1917 to 6.4 years in 1936 in the northern fishery and from 9.3 years in 1922 to 7.6 years in 1936 in the southern fishery.
A Russian scientist, V.M. Borisov reported in 1979 that in the Arcto-Norwegian stocks of Atlantic cod the majority of the fish matured at 8–10 years in the 1930s but by the 1970s the majority of cod matured at 6 years. This decline he attributed to a selective removal of late maturing cod from the population.
Apparently, the MSS-scientists have ignored or haven't read the 1983 Beacham's report from the Scotian shelf in the Northwest Atlantic, where cod catches peaked at over 80 000 tonnes in 1968 and were accompanied by a marked decline in biomass and corresponding decline in mean fish size and age. In addition, the median length and age of cod at maturity declined: the median age at sexual maturity declined from >5 years in the 1960s to < 3 years in 1978 for both males and females. Cod that matured at smaller or younger sizes would have a advantage under heavy fishing pressure as the larger and older maturing cod would be fished out selectively. A similar trend was observed in haddock.
And, in the April 2004 issue of “Nature”, Norwegian fishery scientists E.L.Olsen and his co-authors published a study to the same effect. after having analyzed some 30 years of data involving the plunge in Atlantic cod populations around southern Labrador and Newfoundland's Grand Banks, also wrote that there was a decline in the ages and sizes at maturity even before the northern cod population had collapsed, and that the cause was selective fishing pressure against those larger individuals that genetically tend to mature later.
Not just cod. Dr. Jonas Bjarnason, an Icelandic scientist, explains that due to selective fishing more and more stocks abound with smaller, slower growing fish with inferior reproductive qualities. A genetic change occurs in fish populations, and subsequent age groups mature sooner, spending most of their energy on sexual products and spawning at the expense of body growth. They're more vulnerable to predation and their natural mortality increases
According to Steve Berkeley a research biologist at the University of California in Santa Cruz, the same phenomenon is particularly striking in long-lived rockfish species. “Something is just not right with how we are disproportionately removing older and larger fish " - he reported to a meeting of the American Association for the Advancement of Science (AAAS). "We shouldn't be selectively protecting just the young ones, but adding special protection for the biggest fish. Our research shows that you need to maintain older fish in the population, because those are the most successful at reproducing" - Berkeley said.
See also: http://benyami3.wix.com/benyami - (Go to: Essays in Fisheries Management and Development)
Genetic diversity of marine fisheries resources - Possible impacts of fishing. (FAO)
Evidence for genetic changes due to selective fishing
a. Selection for small size and early maturation in cod Gadus morhua
Figure 1. The median age at sexual maturity for Atlantic cod caught in Fishery Division 4W on the Scotain Shelf, Canada between 1959–79. The traingles joined by a solid line are female data and the circles joined by a dashed line are male data, the dotted line indicates no data available and the vertical bars indicate 95 % confidence limits. (Data from Beacham 1983a).
b. Selection for early maturation in the haddock Melanogrammus aeglefinus
The haddock has been heavily exploited in the demersal fisheries of the northwest Atlantic. Catches of haddock from the St Pierre Bank off Newfoundland rose rapidly to a peak of 58 000 tonnes in 1955, based largely on an exceptional year class in 1949. This large catch was followed by a decline to around 6 000 tonnes in 1957 and further reductions to less than 1 000 tonnes in the 1970s (Templeman and Bishop 1979a). Biological data collected between 1948–51 and 1969–75 show a decline in the mean age at 50% maturity (Fig.2.) from 4.6 to 3.3 years in males and from 5.9 to 4.3 years in females (Templeman and Bishop 1979b). Beacham (1983b) has pointed out that this decline occurred over a period of both increasing and decreasing growth rates, so that the change in age at maturity is not simply related to changes in growth rate due to the compensatory effect of reduced biomass, but is most likely genetic.
Figure 2. Trends in the mean age at 50% sexual maturity for male and female haddock on St Pierre Bank, Canada between 1948–75. The vertical bars represent the 95% confidence limits. (Data from Templeman and Bishop 1979b).
On the Scotian Shelf haddock catches peaked in 1965 at more than 50 000 tonnes and subsequently declined to less than 11 000 tonnes in 1967. Over the same time period the median length at sexual maturity was relatively constant but the median age at maturity declined between 1959–64 and 1975–79 from 3.9 to 2.7 years for males and from 4.4 to 3.0 for females (Beacham 1983c).
----------------------------------------------------------------------------------------------------
See also: http://benyami3.wix.com/benyami - (Go to: Essays in Fisheries Management and Development)
