The following reference is presented in its entirety as it summarizes the unsustainable
practice of many global fisheries.
The research of Dr. Pauly of UBC is an example here of our Marine people profiles.
The problem of by-catch is essential in dealing with sustainable fisheries.
1. Fishing Down Marine Food Webs http://naturalscience.com/ns/cover/cover6.html
2. Marine Food Webs
http://oceanworld.tamu.edu/resources/oceanography-book/marinefoodwebs.htm
This is an online textbook with several graphics illustrating the point. See the
“fishing down marine food web” diagram at the end of page.2.
Fishing Down Marine Food Webs, Daniel Pauly, * Villy
Christensen, Johanne Dalsgaard, Rainer Froese, Francisco Torres Jr.
Science: 6 February 1998 Vol. 279. no. 5352, pp. 860- 863
The mean trophic level of the species groups reported in Food and Agricultural
Organization global fisheries statistics declined from 1950 to
1994. This reflects a gradual transition in landings from long-lived,
high trophic level, piscivorous bottom fish toward short-lived, low
trophic level invertebrates and planktivorous pelagic fish. This effect,
also found to be occurring in inland fisheries, is most pronounced in the
Northern Hemisphere. Fishing down food webs (that is, at lower trophic
levels) leads at first to increasing catches, then to a phase transition
associated with stagnating or declining catches. These results indicate
that present exploitation patterns are unsustainable.
D. Pauly and J.& Dalsgaard, Fisheries Centre, 2204 Main Mall, University
of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4.
V. Christensen, R.Froese, F.Torres Jr.,
International Center for Living Aquatic Resources Management, M.C. Post
Office Box 2631,0718 Makati, Philippines.
Exploitation of the ocean for fish and marine invertebrates, both wholesome and valuable
products, ought to be a prosperous sector, given that capture
fisheries–in contrast to agriculture and aquaculture–reap harvests that
did not need to be sown. Yet marine fisheries are in a global crisis,
mainly due to open access policies and subsidy-driven over-capitalization(1) It may be argued, however, that the global crisis is mainly one of economics or of governance, whereas the global resource base itself fluctuates naturally. Contradicting this more optimistic view, we show here that landings from global fisheries have shifted in the last 45 years from large piscivorous fishes toward smaller invertebrates and planktivorous fishes, especially in the Northern Hemisphere. This may imply major changes in the structure of marine food webs. Two data sets were used. The first has estimates of trophic levels for 22 different species or groups of fish and invertebrates, covering all statistical categories included in the official Food and Agricultural Organization (FAO) landings statistics(2). We obtained these estimates from 60 published mass-balance trophic models that covered all major aquatic ecosystem types(3, 4). The models were constructed with the Ecopath software(5) and local data that included detailed diet compositions(6). In such models, fractional trophic levels (7) are estimated values, based on the diet compositions of all ecosystemcomponents rather than assumed values; hence, their precision and accuracy
are much higher than for the integer trophic level values used in earlierglobal studies (8). The 22 trophic levels derived from these6 Ecopath applications range from a definitional value of 1 forprimary producers and detritus to 4.6 (± 0.32) for snappers(family Lutjanidae) on the shelf of Yucatan, Mexico (9). The second dataset we used comprises FAO global statistics (2) of fisheries landings forthe years from 1950 to 1994 which are based on reportssubmitted annually by FAO member countries and other states and wererecently used for reassessing world fisheries potential (10). By combiningthese data sets we could estimate the mean trophic level of landings,presented here as time series by different groupings of all FAO statistical areas and for the world (11). For all marine areas,
the trend over the past 45years has been a decline in the mean trophic level of the fisheries landings, from slightly more than 3.3 in the early 1950s to less than 3.1 in 1994 (Fig. 1A). A dip in the 1960s and early 1970s occurred because of extremely large catches
(106 metric tons (t) per year) of Peruvian anchoveta with a low trophic level (12) of 2.2(±0.42). Since the collapse of the Peruvian anchoveta fishery in 1972-1973, the global trend in the trophic level of marine fisheries landings has been one of steady
decline. Fisheries in inland waters exhibit, on the global level, a similar trend as for the marine areas (Fig. 1B): A clear decline in average trophic level is apparent from the early 1970s, in parallel to, and about 0.3 units below, those of marine catches. The previous
plateau, from 1950 to 1975, is due to insufficiently detailed fishery statistics for the earlier decades (10).Fig. 1. Global trends of mean trophic level of fisheries landings, 1950 to 1994. (A) Marine areas; (B) inland areas. [View Larger Version of this Image (13K GIF file)] In northern temperate areas where the fisheries are most developed, the mean trophic level of the landings has declined steadily over the last two decades. In the North Pacific (FAO areas 61 and 67; Fig. 2A), trophic levels peaked in the early 1970s and have since then
decreased rapidly in spite of the recent increase in landings of Alaska
pollock, Theragra chalcogramma, which has a relatively high trophic level
of 3.8 (±0.24). In the Northwest Atlantic (FAO areas 21and 31; Fig. 2B), the fisheries were initially dominated by planktivorous menhaden, Brevoortia spp., and other small pelagics at low trophic levels. As their landings decreased, the average trophic level of
the fishery initially increased, then in the 1970s it reversed to a steep
decline. Similar declines are apparent throughout the time series for the
Northeast Atlantic (FAO area 27; Fig. 2C) and the Mediterranean (FAO area
37; Fig. 2C), although the latter system operates at altogether lower trophic levels.
Fig. 2. Trends of mean
trophic level of fisheries landings in northern temperate areas,
1950 to 1994. (A) North Pacific (FAO areas 61and 67); (B)
Northwest and Western Central Atlantic (FAO areas 21 and 31); (C)
Northeast Atlantic (FAO area 27); and (D) Mediterranean (FAO area 37).
[View Larger Version of this Image (13K GIF file)]
The Central Eastern Pacific (FAO area 77; Fig. 3A), Southern
and Central Eastern Atlantic (FAO areas 41,47,and 34; Fig.
3B), and the Indo-Pacific (FAO areas 51,57,and 71; Fig. 3C)
show no clear trends over time. In the southern Atlantic this is probably
due to the development of new fisheries, for example, on the Patagonian
shelf, which tends to mask declines of trophic levels in more developed
fisheries. In the Indo-Pacific area, the apparent stability is certainly
due to inadequacies of the statistics, because numerous accounts exist
that document species shifts similar to those that occurred in northern
temperate areas (13)
Fig. 3. Trends of mean trophic levels of fisheries landings in the intertropical belt and
adjacent waters. (A) Central Eastern Pacific (FAO area 77); (B) Southwest,
Central Eastern, and Southeast Atlantic (FAO areas 41,34,and
47); and (C) Indo (west)- Pacific (FAO areas 51,57,and 71).
[View Larger Version of this Image (10K GIF file)] The South Pacific areas (FAO areas 81and 87; Fig. 4A) are
interesting in that they display wide-amplitude fluctuations of trophic
levels, reflecting the growth in the mid-1950s of a huge industrial
fishery for Peruvian anchoveta. Subsequent to the anchoveta fishery
collapse, an offshore fishery developed for horse mackerel, Trachurus
murphyi, which has a higher trophic level (3.3 ± 0.21) and whose
range extends west toward New Zealand (14). Antarctica (FAO areas
48 58 and 88; Fig. 4B) also exhibits high-amplitude variation
of mean trophic levels, from a high of 3.4 due to a fishery that
quickly depleted local accumulations of bony fishes, to a low of
2.3 due to Euphausia superba (trophic level 2.2;±0.40), a
large krill species that dominated the more recent catches.
Fig. 4. High-amplitude changes of mean trophic
levels in fisheries landings. (A) South Pacific (FAO areas 81and
87); (B) Antarctica (FAO areas 48,58,and 88). [View Larger
Version of this Image (8K GIF file)]
The
gross features of the plots in Figs. through 4 while consistent with
previous knowledge of the dynamics of major stocks, may provide new
insights on the effect of fisheries on ecosystems. Further interpretation
of the observed trends is facilitated by plotting mean trophic levels
against catches. For example, the four systems in Fig. 5 illustrate
patterns different from the monotonous increase of catch that may be
expected when fishing down food webs (15). Each of the four systems in
Fig. 5 has a signature marked by abrupt phase shifts. For three of the
examples, the highest landings are not associated with the lowest trophic
levels, as the fishing-down-the-food-web theory would predict. Instead,
the time series tend to bend backward. The exception (where landings
continue to increase as trophic levels decline) is the Southern Pacific
(Fig. 5C), where the westward expansion of horse mackerel fisheries is
still the dominant feature, thus masking more local effects.
Fig. 5. Plots of mean trophic levels in fishery
landings versus the landings (in millions of metric tons) in four marine
regions, illustrating typical backward-bending signatures (note variable
ordinate and abcissa scales). (A) Northwest Atlantic (FAO area 21); (B)
Northeast Atlantic (FAO area 27); (C) Southeast Pacific (FAO area 87); (D)
Mediterranean (FAO area 37). [View Larger Version of this Image (18K GIF
file)] The backward-bending feature of
the plots of trophic levels versus landings, which also occurs in areas
other than those in Fig. 5, may be due to a combination of the following:
(i) artifacts due to the data, methods, and assumptions used; (ii) large
and increasing catches that are not reported to FAO; (iii) massive
discarding of bycatches (16) consisting predominantly of fish with low
trophic levels; (iv) reduced catchability as a result of a decreasing
average size of exploitable organisms; and (v) fisheries-induced changes
in the food webs from which the landings were extracted. Regarding item
(i), the quality of the official landing statistics we used may be seen as
a major impediment for analyses of the sort presented here. We know that
considerable under- and misreporting occur (16). However, for our
analysis, the overall accuracy of the landings is not of major importance,
if the trends are unbiased. Anatomical and functional considerations
support our assumption that the trophic levels of fish are conservative
attributes and that they cannot change much over time, even when ecosystem
structure changes (17). Moreover, the increase of young fish as a
proportion of landings in a given species that result from increasing
fishing pressure would strengthen the reported trends, because the young
of piscivorous species tend to be zooplanktivorous (18) and thus have
lower trophic levels than the adults. Items (ii) and (iii) may be more
important for the overall explanation. Thus, for the Northeast Atlantic,
the estimated (16) discard of 3.7 106 t year1 of bycatch would
straighten out the backward-bending curve of Fig. 5B. Item (iv)
is due to the fact that trophic levels of aquatic organisms are inversely
related to size (19). Thus, the relation between trophic level and catch
will always break down as catches increase: There is a lower size limit
for what can be caught and marketed, and zooplankton is not going to be
reaching our dinner plates in the foreseeable future. Low catchability due
to small size or extreme dilution (<1 g m3) is, similarly, a major
reason why the huge global biomass (109 t) of lanternfish (family
Myctophidae) and other mesopelagics (20) will continue to remain latent
resources. If we assume that fisheries tend to switch from
species with high trophic levels to species with low trophic levels in
response to changes of their relative abundances, then the
backward-bending curves in Fig. 5 may be also due to changes in ecosystem
structure, that is, item (v). In the North Sea, Norway pout, Trisopterus
esmarkii, serves as a food source for most of the important fish species
used for human consumption, such as cod or saithe. Norway pout is also the
most important predator on euphausiids (krill) in the North Sea (3). We
must therefore expect that a directed fishery on this small gadoid
(landings in the Northeast Atlantic are about 3 ? 105 t year1)
will have a positive effect on the euphausiids, which in turn prey on
copepods, a much more important food source for commercial fish species
than euphausiids. Hence, fishing for Norway pout may have a cascading
effect, leading to a build-up of nonutilized euphausiids. Triangles such
as the one involving Norway pout, euphausiids, and copepods, and which may
have a major effect on ecosystem stability, are increasingly being
integrated in ecological theory (21), especially in fisheries biology
(22). Globally, trophic levels of fisheries landings appear to
have declined in recent decades at a rate of about 0.1 per decade,
without the landings themselves increasing substantially. It is likely
that continuation of present trends will lead to widespread fisheries
collapses and to more backward-bending curves such as in Fig. 5, whether
or not they are due to a relaxation of top-down control (23). Therefore,
we consider estimations of global potentials based on extrapolation of
present trends or explicitly incorporating fishing-down-the-food-web
strategies to be highly questionable. Also, we suggest that in the next
decades fisheries management will have to emphasize the rebuilding of fish
populations embedded within functional food webs, within large “no-take”
marine protected areas (24)
2. Sequential
megafaunal collapse in the North Pacific Ocean: An ongoing legacy of
industrial whalingfrom:http://www.pnas.org/cgi/content/full/100/21/12223
Published online , 2003,
Abstract:
Populations of seals, sea lions, and sea otters have
sequentially collapsed over large areas of the northern North Pacific
Ocean and southern Bering Sea during the last several decades. A bottom-up
nutritional limitation mechanism induced by physical oceanographic change
or competition with fisheries was long thought to be largely responsible
for these declines. The current weight of evidence is more consistent with
top-down forcing. Increased predation by killer whales probably drove the
sea otter collapse and may have been responsible for the earlier pinniped
declines as well. We propose that decimation of the great whales by
post-World War II industrial whaling caused the great whales’ foremost
natural predators, killer whales, to begin feeding more intensively on the
smaller marine mammals, thus “fishing-down” this element of the marine
food web. The timing of these events, information on the abundance, diet,
and foraging behavior of both predators and prey, and feasibility analyses
based on demographic and energetic modeling are all consistent with this
hypothesis.
The abrupt decline of the
western stock of Steller sea lions (Eumetopias jubatus) across most of
the northern North Pacific Ocean and southern Bering Sea is one of the
world’s most well known yet poorly understood marine conservation
problems. For years, scientists attributed this decline to nutritional
limitation, the presumed consequence of a climate regime shift and/or
competition with regional fisheries (1). Although fisheries and regime
shifts undoubtedly influenced both the fishes and their associated food
webs (2–5), several recent reviews of the available information on sea
lions and their environment, including an assessment by the National
Research Council, cast doubt on the nutritional limitation hypothesis (6,
7), notwithstanding evidence from field and laboratory studies that diet
quality is a factor in sea lion energetics (8). The doubt stems from three
main findings. First, most measures of behavior, physiology, and
morphology from surviving adult sea lions and pups in the western Gulf of
Alaska and Aleutian Islands are inconsistent with nutritional limitation.
These animals have better body condition, reduced foraging effort, and
reduced field metabolic rates relative to similar measures from the
increasing sea lion population in southeast Alaska (7). Second, sea lion
prey is abundant in most areas of the decline (9). Known changes in prey
availability and other features of the oceanic ecosystem are particularly
incongruous with the most precipitous phase of the decline, which occurred
during the mid- to late 1980s, and can be accounted for only by greatly
increased adult mortality (6). Third, populations of piscivorous sea
birds, many of which feed on earlier life stages of the same fish species
consumed by sea lions, have remained stable or increased in the same area
and over the same period that the sea lions have declined (10). Top-down
forcing now appears to have been an important contributor to declines of
Steller sea lions and other marine mammal populations in the region (6).
Likely top-down forcing factors include purposeful shooting, incidental
mortality in fishing gear, and predation. We will suggest that increased
predation was paramount among these factors, and that altered food web
dynamics brought about by human overharvesting initiated the
change.
In the
report “Progress Towards Environmental Sustainability in British
Columbia’s Seafood Sector., May 2001” there are a number of excellent
graphics which present a framework for sustainable fisheries.
http://www.bcseafoodalliance.com/BCSA/AMRSummitReport.pdf
5.4 The Precautionary Principle:
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