Historical changes in mean trophic level of southern Australian fisheries

2014 ◽  
Vol 65 (10) ◽  
pp. 884 ◽  
Author(s):  
Heidi K. Alleway ◽  
Sean D. Connell ◽  
Tim M. Ward ◽  
Bronwyn M. Gillanders
Keyword(s):  
Trophic Level ◽  
Spatial Data ◽  
South Australia ◽  
Trophic Levels ◽  
High Trophic Level ◽  
Fishing Pressure ◽  
Time Line ◽  
The Mean ◽  
Mean Trophic Level ◽  
Catch Statistics

Decreases in the mean trophic level (MTL) of fishery catches have been used to infer reductions in the abundance of high trophic level species caused by fishing pressure. Previous assessments of southern Australian fisheries have been inconclusive. The objectives of the present study were to provide more accurate estimates of MTL using disaggregated taxonomic and spatial data. We applied the model of MTL to fisheries catch statistics for the state of South Australia from 1951 to 2010 and a novel set of historical market data from 1936 to 1946. Results show that from 1951 to 2010, MTL declined by 0.16 of a trophic level per decade; a rate greater than the global average of 0.10 but equivalent to similar regional investigations in other areas. This change is mainly attributable to large increases in catches of sardine, rather than reductions in the catches of high trophic level species. The pattern is maintained when the historical data is included, providing a time line from 1936 to 2010. Our results show a broadening of the catch of lower trophic levels and suggest care in interpretation of MTL of catches because reductions do not necessarily reflect change in high trophic level species by fishing pressure.

Fisheries

2007 ◽  
Author(s):  
John R. Beddington ◽  
Geoffrey P. Kirkwood
Keyword(s):  
Trophic Level ◽  
Global Scale ◽  
Trophic Levels ◽  
Predatory Fish ◽  
Apex Predators ◽  
The North ◽  
The Mean ◽  
Mean Trophic Level ◽  
Global Statistics ◽  
United Nations Food

The depletion of fish stocks on a global scale is well documented. The United Nations Food and Agriculture Organisation collects statistics on fisheries from all states and, despite obvious shortcomings in the data, a clear picture has been available for some time. Garcia and Grainger (2005) have succinctly documented the position from the latest available date: in 2003, only 3% of stocks were underexploited and 26% moderately exploited, while 52% were fully exploited, 16% were overfished, 7% were depleted, and 1% were recovering from earlier depletion. These global statistics mask two important phenomena. The first, highlighted by Pauly et al. (1998), is that fisheries are increasingly focusing on species lower down in the food-web and the second, highlighted by Myers and Worm (2003, 2005), is that large predatory fish have been particularly reduced in abundance. Both of these analyses are somewhat flawed. In the case of Pauly et al. there are two problems: the first is that the metrics used for the mean trophic level are presented as simple numbers with no estimates of error or indeed sensitivity. In such a situation, the changes in mean trophic levels are hard to interpret, particularly where the mean trophic level changes by at most around 10% over four decades. The second problem has been highlighted by a recent paper by Essington et al. (2006). They point out that in the periods when according to the analysis of Pauly et al. the mean trophic level was declining, in most cases catches of apex predators and indeed all upper trophic levels increased (an exception is the North Atlantic). In the case of the Myers and Worm analysis, they used the catch per unit of effort (CPUE) as an index of abundance. As discussed later in this chapter, there are problems with this, but more importantly for some key apex predators, in particular large tunas, the CPUE declines in the early stages of the fishery, where catches are small, but remains relatively stable under a regime of much higher catches. In such a situation, the interpretation that the CPUE reflects changes in abundance is clearly problematic.

scholarly journals Overfishing of marine resources: some lessons from the assessment of demersal stocks off Mauritania

2014 ◽  
Vol 72 (2) ◽  
pp. 414-427 ◽  
Author(s):  
B. Meissa ◽  
D. Gascuel
Keyword(s):  
Trophic Level ◽  
Rapid Development ◽  
Fishing Effort ◽  
Trophic Levels ◽  
Production Model ◽  
Comparative Approach ◽  
Small Scale ◽  
Fishing Pressure ◽  
Ecological Requirements ◽  
The Mean

Abstract The recent, rapid development of fishing in Mauritania offers a good case study for a comparative approach of the resilience of the species exploited there, in the face of increasing fishing pressure. First, we assessed the health of 22 demersal stocks with differing ecological requirements, demographic strategies, and states of exploitation. A dynamic production model was fitted in a framework of Bayesian statistics to abundance indices estimated from scientific trawl surveys or commercial catch per unit efforts. We show that 12 of the 22 stocks assessed are overexploited and 3 are fully exploited. The combined assessment of all 22 stocks demonstrates an overall overexploitation, with total demersal biomass decreasing by ∼75% since 1982 and fishing effort 30% higher than that at maximum sustained yield (40% higher for finfish). Second, relations between states of stocks and life history traits were analysed. The stocks of large and vulnerable species currently undergo the highest fishing pressure and are those that are the most overexploited. At the scale of the community represented by the stocks considered, surveys-based indictors of the mean intrinsic vulnerability, the mean maximum length, and the mean trophic level exhibit a significant decrease from 1990 to 2010. Changes observed in catch-based indicators depend on fishing strategies and are impacted by the recent development of the small-scale fishery. But indicators expressed as a function of a multiplier of fishing effort or fishing mortality clearly decrease, thus confirming that the intensification of exploitation leads to communities dominated by smaller species and lower trophic levels. We conclude that large and high trophic level species, such as white grouper, meagre, guitarfish, and smooth-hound, are markers of ecosystem health and should be considered as sentinel species.

Fishery-induced Inter-annual Changes in the Mean Trophic Level, the Northern Sea of Oman off the Iranian Coast, 2002–2011

2018 ◽  
Vol 53 (4) ◽  
pp. 655-665
Author(s):  
Sakineh Mashjoor ◽  
Fatemeh Heidary Jamebozorgi ◽  
Ehsan Kamrani
Keyword(s):  
Trophic Level ◽  
Iranian Coast ◽  
The Mean ◽  
Mean Trophic Level ◽  
Annual Changes ◽  
Sea Of Oman

Fishing through (and up) Alaskan food webs

2009 ◽  
Vol 66 (2) ◽  
pp. 201-211 ◽  
Author(s):  
Michael A. Litzow ◽  
Daniel Urban
Keyword(s):  
Time Series ◽  
Trophic Level ◽  
High Trophic Level ◽  
Climate Index ◽  
Climate Regime ◽  
Fisheries Sustainability ◽  
Competing Hypotheses ◽  
Mean Trophic Level ◽  
Highly Correlated ◽  
Historical Periods

We used a 112-year time series of Alaskan fishery catches to test competing hypotheses concerning trends in mean catch trophic level, a widely used indicator of fisheries sustainability. We found that mean trophic level has generally remained steady or increased in recent decades on Alaska-wide and regional scales, indicating stable catches of high trophic level taxa. During historical periods of declining mean trophic level, catches of upper trophic level taxa either increased or remained steady, contrary to the predictions of the “fishing down the food web” hypothesis. Further, a climate index was highly correlated (r = 0.69–0.97) with mean trophic level and (or) the related fisheries in balance (FIB) index across climate regime shifts in the 1940s and 1970s, indicating that climate effects, particularly on high trophic level taxa, can act as the major driver of variability in these parameters. These results provide a contrast to the view of ubiquitous declines in mean trophic level of fishery catches, driven by overexploitation and serial stock replacement.

Are We Catching What They Eat? Moving Beyond Trends in the Mean Trophic Level of Catch

Fisheries ◽  
2015 ◽  
Vol 40 (8) ◽  
pp. 376-385 ◽  
Author(s):  
Konstantine J. Rountos ◽  
Michael G. Frisk ◽  
Ellen K. Pikitch
Keyword(s):  
Trophic Level ◽  
The Mean ◽  
Mean Trophic Level

scholarly journals Long-term changes in the mean trophic level of Central Chile fishery landings

2005 ◽  
Vol 69 (2) ◽  
pp. 295-300 ◽  
Author(s):  
Hugo Arancibia ◽  
Sergio Neira
Keyword(s):  
Trophic Level ◽  
Central Chile ◽  
Long Term Changes ◽  
The Mean ◽  
Mean Trophic Level

The Geographical Expansion of Fisheries conceals the decline in the Mean Trophic Level of Iran's catch

2021 ◽  
Vol 199 ◽  
pp. 105411
Author(s):  
Moslem Daliri ◽  
Ehsan Kamrani ◽  
Ali Salarpouri ◽  
Abdulrahman Ben-Hasan
Keyword(s):  
Trophic Level ◽  
Geographical Expansion ◽  
The Mean ◽  
Mean Trophic Level

Community Stability and the Strategy of Biological Control

1965 ◽  
Vol 97 (8) ◽  
pp. 887-895 ◽  
Author(s):  
K. E. F. Watt
Keyword(s):  
Biological Control ◽  
Trophic Level ◽  
Community Organization ◽  
Biological Control Agent ◽  
Trophic Levels ◽  
Food Plants ◽  
Pest Species ◽  
Community Stability ◽  
The Mean ◽  
The Relationship

AbstractCommunity organization is defined as the mean number of trophic links connecting species of different trophic levels in a community. For the special purposes of this paper, competition is assumed to occur whenever two species are known to eat the same food species. Community stability is defined as the reciprocal of the mean, for all species, of the standard error of logarithms of annual collection sizes. It is thus a measure of stability over time of the species populations in a community. Several authors writing about the relationship between community organization and community stability have insisted that the stability of a complex ecological system increases with the number of avenues by which energy can flow through it. This theory does not seem consistent with the observation that some insect pest species of notorious instability are attacked by a great number of entomophagous species. In this paper we seek additional evidence bearing on the relationship between community organization and stability, using computer analysis of data collected by the Canadian Forest Insect Survey on forest Macrolepidoptera and their food plants. After considering our results, and those of other workers, we postulate the following hypothesis. Stability at any herbivore or carnivore trophic level increases with the number of competitor species at that level, decreases with the number of competitor species that feed upon it, and decreases with the proportion of the environment containing useful food. If this hypothesis is valid, too much competition in the entomophagous trophic level will not allow the species in that level to be instable enough to control an unstable pest. Therefore, the best type of biological control agent is one that has no direct competitor species. Also, the most unstable biological control agents, and hence those capable of controlling an unstable pest, will be polyphagous.

Towards coherent GES assessments at sub-regional level: signs of fisheries expansion processes in the Bay of Biscay using an OSPAR food web indicator, the mean trophic level

2019 ◽  
Vol 76 (6) ◽  
pp. 1543-1553 ◽  
Author(s):  
Nina-Larissa Arroyo ◽  
Georges Safi ◽  
Pauline Vouriot ◽  
Lucía López-López ◽  
Nathalie Niquil ◽  
...  
Keyword(s):  
Trophic Level ◽  
Spatial Scales ◽  
Management Strategies ◽  
Regional Level ◽  
Bay Of Biscay ◽  
Ecosystem Studies ◽  
The Mean ◽  
Mean Trophic Level ◽  
Over Exploitation

Abstract Using the Bay of Biscay (BoB) as a case study, we conducted a transnational assessment of the mean trophic level (MTL, Ospar FW4) indicator at sub-regional level, over the last three decades. Our results confirm the apparent recovery of BoB’s bentho-demersal system, as shown by trends in the MTL indicator based on survey data. However, they also point at a concomitant “fishing through” process where the apparent stability revealed by the MTL indicator based on landed catch data may be masking the expansion of demersal fisheries to deeper waters, and an over-exploitation of resources (particularly abundant pelagic species). Moreover, they show how the combined examination of independent surveys and fishery landings allows the identification of ecological trends in ecosystem studies. In addition, our results confirm that analysing MTL at various threshold levels helps discerning the causality of trends in this indicator, especially if analyses for pelagic and demersal species are run independently. Further studies, at smaller (i.e. local) spatial scales, need to be conducted to ascertain our results and suggest appropriate management strategies aimed at regulating fisheries expansions in the area.

The mean trophic level of Uruguayan landings during the period 1990–2001

2005 ◽  
Vol 74 (1-3) ◽  
pp. 223-231 ◽  
Author(s):  
Andrés C. Milessi ◽  
Hugo Arancibia ◽  
Sergio Neira ◽  
Omar Defeo
Keyword(s):  
Trophic Level ◽  
The Mean ◽  
Mean Trophic Level
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