Large size can be important for a number of reasons, but so can the smaller sizes. The fishing economy (freshwater) around Lake Okechobee was estimated to be more than $117 million during 2000, and is a major portion of the local and state economy. On Lake Okeechobee a few years back, FL state wildlife managers became aware a decrease in the black bass population. This became clear to them through meetings with angler clubs, conducting creel surveys, along with actual monitoring The population was exhibiting a disproportionate number of "large fish", while they were simultaneously not seeing as many of the "medium sized fish".

It was concluded that this was an indicator of lack of vegetation/cover. The smaller fish had some vegetation near the shore in which to hide, but once they reached a certain size they lacked adequate cover and the predation pressure was significant. This was a concern for a number of reasons, as over time that would mean fewer fish overall, fewer would recruit into the large fish slot and the potential for impacts to the quality of the gene pool. They have worked to remedy this situation. Here is a general link about what they are doing now.

http://www.eregulations.com/florida/fishing/freshwater/black-bass-plan-update/

The whole food web can be modified by predatory fish even to water quality. On the other hand, changes in lower levels can affect the number of top predators, for examples disturbances in reproduction or increasing competition in juvenile stage. However, I think that in most cases, at least in lakes, the proportion of large fish is regulated by the intensity of fishing focusing to these species or individuals.

Dear Nathalie,

could you share a link or a paper about this Large Fish Index? I do not know it.

I do not know either wether you know these papers or not but I think both could be very helpful in this topic for everybody. Both papers underline the importance of big fishes in the food web and in the ecosystem.

Here comes the first:

And this is the second I mentioned:

You might want to try the scaled mass index as an alternative ecosystem health indicator, as explained in this recent paper in Freshwater Biology:

https://www.researchgate.net/publication/260342483_Scaled_mass_index_shows_how_habitat_quality_influences_the_condition_of_four_fish_taxa_in_north-eastern_Spain_and_provides_a_novel_indicator_of_ecosystem_health?ev=prf_pub

Article Scaled mass index shows how habitat quality influences the c...

Should work! Have a look to these references that study the effects of drought or warming in stream food webs. I have just read them and they might be handy for you:

Ledger, M. E., L. E. Brown, F. K. Edwards, A. M. Milner, and G. Woodward. 2012. Drought alters the structure and functioning of complex food webs. Nature Climate Change 3:223-227.

Woodward, G., L. E. Brown, F. K. Edwards, L. N. Hudson, A. M. Milner, D. C. Reuman, and M. E. Ledger. 2012. Climate change impacts in multispecies systems: drought alters food web size structure in a field experiment. Philosophical Transactions of the Royal Society of London 367:2990–2997.

You should try for food web topology; where you should see different linkages should be assessed. Knowing these links is important before we may reach to any conclusion whether any organism is sensitive or not.

Dear Nathalie

in order to characterize a food web the number of species having a given size may be equally or even more important, given that LFI often depends on a single species. In turn, Food web structure depends on its complexity , and therefore indices that are able to reflect functional diversity could apply better - I used an index called phi40, see Fock HO, Kloppmann M, Probst WN (2014) An early footprint of fisheries: changes for a demersal fish assemblage in the German Bight from 1902-1932 to 1991-2009. Journal of Sea Research 85:325-335

Dear Nathalie,

We used that in the paper on the MSFD in the Basque coast, published in 2011. You know better than me that, in principle, healthy food-webs would have larger fish on the top of the web. Hence, many scientists have supported the idea of using such indicator when looking for the environmental status. In turn, there are several papers dealing with climate change in which it seems that in temperate waters future fish scenarios show assemblages of fish species smaller in size. Hence, this means that if the objective for the large fish composition is X, this would never been achieved, since in the future the assemblage composition would be completely different. This is the unending discussion on static and dynamic targets.... We can discuss in DEVOTES project

Yes, I think that the Large Fish Index is very important indicator for community structure. Few big size fishes underline that the fish assemblage is represented especially by juveniles individuals with scarce presence of spawners indicanting an antropic pressure in the area (for ex. fishing pression).

A large fish index has been considered as an indicator of food web or community structure, and of exploitation. However, we must be careful with equating large fish to ecosystem health. a drop in large fish index reflects exploitation impacts and targets, but not health. Some organisations may think that an exploited ecosystem is less healthy than an exploited one, but i would certainly argue this is a normative statement.

The idea is good. Large Fish Index is related to general index of asymmetric (Skewness) distribution in populations. This indicator shows general state of growth of any population.

Similar indices that incorporate fish biomass into community assessments have been used in freshwater ecosystems.

Gammon, J.R. 1976. The fish population of the middle 340km of the Wabash River. Purdue University Water Resources Research Center, LaFayette, Indiana. Technical Report 86.

Gammon, J.R. 1980. The use of community parameters derived from electrofishing catches of river fish as indicators of environmental quality, in Seminar on Water Quality Management Tradeoffs. U.S. Environmental Protection Agency, Washington, D.C. EPA-905/9-80-009.

Hughes, R.M. and J.R. Gammon. 1987. Longitudinal changes in fish assemblages and water quality in the Willamette River, Oregon. Transactions of the American Fisheries Society 116(2):196-209.

I think this paper can be of interest for you Urban, Mark C.BГјrger, Reinhar, Bolnick, Daniel I. Asymmetric selection and the evolution of extraordinary defences. Nat Commun 2013/07/02/online, V4, http://dx.doi.org/10.1038/ncomms3085

Hi Nathalie,

At an ICES meeting of the Working Group for Fish Ecology, Simon Greenstreet led an analysis of a range of metrics to determine how best to characterise marine communities, identifying redundancy and reducing the number of potential metrics, including LFI, to a minimum number that best described the ecosystem:

http://icesjms.oxfordjournals.org/content/69/1/8.full

Greenstreet SPR, Fraser HM, Rogers SI, Trenkel VM, Simpson SD, Pinnegar JK (2012) Redundancy in metrics describing the composition, structure and functioning of the North Sea demersal fish community. ICES Journal of Marine Science 69:8-22. doi: 10.1093/icesjms/fsr188

Hope that is useful to your work.

Cheers,

Steve Simpson

Dear,

it might give valuable information, also on single-species level. However, the index is rather sensitive (c.f. Sunderlöf et al. 2014) for small inconsistencies in data collection and binning. The index is also very sensitive for changes in growth. The index increases automatically with improved growth conditions and vice versa and mortality rates (i.e. abundance) is not the only factor involved.

Thank you very much to you all. Your answers and related publications were of great help !

I was going to suggest that there were myriad examples from freshwater sciences that might be suitably adapted to address marine condition when I found this:

http://ftp.sccwrp.org/pub/download/TOOLS/SQO/BenthicIndexCalculation_Embayments_01_21_08.pdf

Determining Benthic Invertebrate Community Condition in Embayments

INTRODUCTION

Research in California embayments has shown that the use of a combination of benthic indices provides a more accurate description of benthic invertebrate community condition than does the use of a single index (Ranasinghe et al. 2007). This document describes the steps necessary to calculate four benthic indices:

1. Index of Biotic Integrity (IBI);

2. Relative Benthic Index (RBI);

3. Benthic Response Index (BRI); and

4. River Invertebrate Prediction and Classification System (RIVPACS).

Hello dear

can iyou send me this paper ?

best wishes

Hi Nathalie, You may want to consider to the potential effect of global warming on the size of fish when using LFI. Have a look at these ref. to have a better idea of climate change on size of fish.

Daufresne, M., Lengfellner, K. and Sommer, U., 2009. Global warming benefits the small in aquatic ecosystems. Proceedings of the National Academy of Sciences of the United States of America 106, 12788-12793.

Hessen, D.O., Daufresne, M. and Leinaas, H.P., 2013. Temperature-size relations from the cellular-genomic perspective. Biological Reviews 88, 476-489.

Hi Nathalie

as a food web AND ecosystem health indicator, LFI would be more usefull if it would include also diversity, see

Fock HO, Kloppmann M, Probst WN (2014) An early footprint of fisheries: changes for a demersal fish assemblage in the German Bight from 1902-1932 to 1991-2009. Journal of Sea Research 85:325-335

All the best Heino

Thank you very much for these answers. They are all very compmlementary !