Polet, H.

Rijnsdorp, A.D.

Stamoulis, A.

Torreele, E.

Open Marine Archive

  
ICES CM 2014/G:25 
The Large Fish Indicator in North Sea waters: Does it respond to the recent decline 
in fishing? 
 
Antonios Stamoulis (1,2), Hans Polet (1), Els Torreele (1) and Adriaan D. Rijnsdorp (3,2) 
(1) The Institute for Agricultural and Fisheries Research (ILVO), Animal Sciences – Fisheries, Ankesrstraat 1, 
8400 Ostend, Belgium. (2) Aquaculture and Fisheries group, Wageningen University, Wageningen, The 
Netherlands. (3) Wageningen IMARES, Institute for Marine Resources and Ecological Studies, IJmuiden, The 
Netherlands. Presenter contact details: antonios.stamoulis@gmail.com, Phone +3259569834 
 
Summary 
The Large Fish Indicator (LFI) describes changes in the proportion of large fish at community level 
and gives an indication about its state. In the North Sea (>40cm total length), although proposed as an 
indicator of Good Environmental Status, the LFI may vary between different regions. Using data from 
the International Bottom Trawl Survey (IBTS) and Beam Trawl Survey (BTS), we investigated the LFI 
response to recent changes in fishing mortality rates. To examine changes in LFI trends, we calculated 
the indicator for different areas of the North Sea. The indicator responded to changes in fishing 
mortality but mainly based on BTS data. Moreover, when calculated with IBTS data, the indicator was 
highly influenced by single species contribution such as cod, underpinning that the LFI might be more 
sensitive to other pressures than fishing. A suite of indicators covering different data, dimensions and 
processes would elucidate trends and rapid changes of the LFI. The BTS survey can provide a reliable 
sound basis of information for calculating the LFI in the southern North Sea waters. 
 
Introduction 
Almost all fisheries are size-selective by nature. Size-dependent harvesting reduces the abundance of 
larger fish and those that have the potential to grow large. Size of fish, however, is a primary factor in 
ecological processes (Shin et al., 2005). The Large Fish Indicator (LFI) describes changes in the 
proportion (by weight) of the fish community that is larger than a specified length threshold (40cm in 
the North Sea) (Greenstreet et al., 2010). Fishing pressure in the North Sea is applied mainly by two 
bottom-trawling fishing gears: a) the beam trawl that targets primarily flatfish and is more common in 
the southern North Sea, and b) the otter trawl that is more common in the northern North Sea and 
targets mainly roundfish. It is known that fishing mortality in the North Sea has been declining. We 
calculated LFI trends in different areas of the North Sea and examined whether the indicator reflects 
changes in fishing mortality. 
Materials and Methods 
We analysed data from the ICES International Bottom Trawl Survey (NS-IBTS) and Beam Trawl 
Survey (BTS) collected during the first (Q1) and third (Q3) quarter of the year respectively. As an area 
of study, we defined spatial subsets of the North Sea (Greater NS, Northern NS, Southern NS), based 
on the design of each survey, with IBTS extending to the greater North Sea area and BTS focusing on 
the southern part. We used ICES statistical rectangles for reporting; statistical rectangles sampled less 
  
than 50% of all years were excluded to reduce spatial bias. The north-south division of the North Sea 
was defined as described by Fraser et al. (2008). We calculated the LFI following the protocol proposed 
by Greenstreet et al. (2010), including only species defined as demersal according to Engelhard et al. 
(2010). North Sea’s fishing mortality rates, available from ICES stock assessments, delivered demersal 
roundfish (RoundfishFcom) and flatfish (FlatfishFcom) community’s averaged mortality rates. We 
investigated the statistical correlations between two groups, RoundfishFcom and IBTS-LFI, and 
FlatfishFcom along with BTS-LFI. Moreover, we applied time-lags of >10 years to examine the long-
term relationship between LFI and fishing pressure. 
Results and Discussion 
IBTS-LFI 40cm trends for the greater North Sea were found being comparable to those reported in the 
literature (Greenstreet et al., 2010). The 40cm BTS-LFI trends were strongly inversely correlated to 
changes in the fishing mortality applied on flatfish species. However, correlation for BTS-LFI in the 
northern NS was not statistically significant. BTS-LFI trends for most regions in study showed a time-
lagged response to changes of fishing mortality between 1 to 5 years. 
The IBTS-LFI series were not correlated to fishing mortality except the one calculated in the Southern 
Part of the North Sea (SPNS) (Figure 1A), which was correlated positively, implying that an increase 
in fishing mortality would be followed by an increase in the LFI. 
A possible explanation for this paradox is the high biomass 
contribution of a single species, cod, the abundance of which has 
been declining during recent times, as it is known that its centre 
of distribution to be shifting towards northern North Sea waters. 
When excluding cod, the aforementioned correlation was not 
significant. 
Therefore, LFI trends calculated with BTS data did reflect the 
recent decline of fishing effort, especially in the southern waters 
(Figure1B), whereas the IBTS-based ones did not. The BTS-LFIs 
were substantially lower than the IBTS-LFIs for the same regions 
(data not shown). This confirms the facts that SPNS is hosting 
smaller fish than the northern, as well as the increased 
catchability of the BTS towards smaller individuals.  
A suite of indicators covering different data, dimensions and 
processes would elucidate trends and rapid changes of the LFI. 
The BTS survey can provide a reliable sound basis of information 
for calculating the LFI in the southern North Sea waters. 
References 
Engelhard, G. H. G., Ellis, J. J. R., Payne, M. R., ter Hofstede, R., and Pinnegar, J. K. 2010. Ecotypes as a concept for 
exploring responses to climate change in fish assemblages. ICES Journal of Marine Science, 68: 580–591. 
Fraser, H. M., Greenstreet, S. P. R., Fryer, R. J., and Piet, G. J. 2008. Mapping spatial variation in demersal fish 
species diversity and composition in the North Sea: accounting for species- and size-related catchability in 
survey trawls. ICES Journal of Marine Science, 65 : 531–538.  
Greenstreet, S. P. R., Rogers, S. I., Rice, J. C., Piet, G. J., Guirey, E. J., Fraser, H. M., and Fryer, R. J. 2010. 
Development of the EcoQO for the North Sea fish community. ICES Journal of Marine Science, 68: 1–11.  
Shin, Y., Rochet, M., Jennings, S., Field, J., and Gislason, H. 2005. Using size-based indicators to evaluate the 
ecosystem effects of fishing. ICES Journal of Marine Science, 62: 384–396.  
Figure 1: LFI-Fcom charts for (A) LFI 
IBTS SPNS and roundfish Fcom, (B) 
LFI BTS SPNS and flatfish Fcom. In 
both cases LFI is calculated for 40cm. 


The Large Fish Indicator in North Sea waters: does it respond to the recent declinein fishing?

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