There are several important features in the estimated time series of bigeye recruitment (Figure D-2).

Figure D-2: Estimated annual recruitment of bigeye tuna to the fisheries of the EPO. The estimates are scaled so that the estimate of virgin recruitment is equal to 1.0 (dashed horizontal line). The solid line shows the maximum likelihood estimates of recruitment, and the shaded area indicates the approximate 95% intervals around those estimates.

The most obvious pattern is a period of below-average recruitment prior to 1994, which may be a model artifact caused by the expansion of the floating-object fisheries in 1994. Estimates of recruitment before 1993 are more uncertain, as the floating-object fisheries were not catching significant amounts of small bigeye. There was a period of above-average annual recruitment in 1994-1998, followed by a period of below-average recruitment in 1999-2000. The recruitments were above average from 2001 to 2006, and were particularly high in 2005 and 2006. The 2008 and 2010 recruitments were above average, but the recruitment in 2011 appears to have been below average. However, this recent estimate is very uncertain and should be regarded with caution, due to the fact that recently-recruited bigeye are represented in only a few length-frequency samples.

Over the range of spawning biomasses estimated by the base case assessment, the abundance of bigeye recruits appears to be unrelated to the spawning potential of adult females at the time of hatching.

There have been important changes in the amount of fishing mortality caused by the fisheries that catch bigeye tuna in the EPO. On average, since 1993 the fishing mortality of bigeye less than about 15 quarters old has increased substantially, and that of fish more than about 15 quarters old has also increased but to a lesser extent (Figure D-3)

Figure D-3: Total catches (retained catches plus discards) of bigeye tuna by the purse-seine fisheries, and retained catches for the longline fisheries, in the eastern Pacific Ocean. The purse-seine catches are adjusted to the species composition estimate obtained from sampling the catches. The 2011 catch data are preliminary.

and (Figure D-4).

Figure D-4: Average annual fishing mortality, by all gears, of bigeye tuna recruited to the fisheries of the EPO. Each panel illustrates the average fishing mortality rates that affected the fish within the range of ages indicated in the title of each panel. For example, the trend illustrated in the top panel is an average of the fishing mortalities that affected the fish that were 1-4 quarters old.

The increase in the fishing mortality of the younger fish was caused by the expansion of the purse-seine fisheries that catch tuna in association with floating objects. It is clear that the longline fishery had the greatest impact on the stock prior to 1995, but with the decrease in longline effort and the expansion of the floating-object fishery, at present the impact of the purse-seine fishery on the population is far greater than that of the longline fishery (Figure D-5).

Figure D-5: Trajectory of the spawning biomass of a simulated population of bigeye tuna that was not exploited (top line) and that predicted by the stock assessment model (bottom line). The shaded areas between the two lines show the portions of the impact attributed to each fishing method. t = metric tons.

The discarding of small bigeye has a small, but detectable, impact on the depletion of the stock.

Since the start of 2005, when the spawning biomass ratio (the ratio of the spawning biomass at that time to that of the unfished stock; SBR) was at its historic low level of 0.16, the bigeye stock has shown a recovery trend, to an SBR of 0.24 at the end of 2010. This recent recovery trend is subsequent to the IATTC tuna conservation resolutions initiated in 2004. The SBR is estimated to have declined slightly since the beginning of 2011 to a level of 0.23 at the start of 2012 (Figure D-6).

Figure D-6: Estimated spawning biomass ratios (SBRs) for bigeye tuna in the EPO. The dashed horizontal line (at about 0.20) identifies the SBR at MSY. The solid curve illustrates the maximum likelihood estimates, and the estimates after 2012 (large dot) indicate the SBR predicted to occur if fishing mortality rates continue at the average of that observed during 2009-2011, and average environmental conditions occur during the next 10 years. The shaded area indicates the approximate 95% confidence intervals around those estimates.

According to the base case model, this most recent SBR is about 12% higher than the maximum sustainable yield (MSY) level (Table D-1).

Recent catches are estimated to have been 8% less than those corresponding to the MSY levels (Table D-1).

If fishing mortality (F) is proportional to fishing effort, and the current patterns of age-specific selectivity are maintained, the level of fishing effort corresponding to the MSY is about 95% of the current (2009-2011) level of effort (Table D-1).

According to the base case results, the two most recent estimates indicate that the bigeye stock in the EPO is probably not overfished (S>S_MSY), but that fishing mortality slightly exceeds the level corresponding to the MSY (overfishing is taking place, F>F_MSY) (Figure D-7). This interpretation, however, is subject to uncertainty, as indicated by the approximated confidence intervals around the most recent estimate in the Kobe plot (Figure D-7).

Figure D-7: Kobe (phase) plot of the time series of estimates of stock size (top: spawning biomass; bottom: total biomass) and fishing mortality relative to their MSY reference points. Each dot is based on the average fishing mortality rate over three years; the large dot indicates the most recent estimate. The squares around the most recent estimate represent its approximate 95% confidence interval.

The MSY of bigeye in the EPO could be maximized if the age-specific selectivity pattern were similar to that of the longline fisheries, because they catch larger individuals that are close to the critical weight. Before the expansion of the floating-object fishery that began in 1993, the MSY was greater than the current MSY and the fishing mortality was less than F_MSY (Figure D-8).

Figure D-8: Estimates of MSY-related quantities calculated using the average age-specific fishing mortality for each year. (S(recent) is the spawning biomass at the end of the last year in the assessment.)

Under the current levels of fishing mortality, recent spikes in recruitment are predicted not to sustain the increasing trend observed for SBR since 2004. Both the base case and the assessment assuming a stock-recruitment relationship indicate a slight decline of the SBR since the start of 2011 and predict that the population is likely to further drop below the level corresponding to MSY under average recruitment conditions (Figure D-6). It is estimated that catches will be lower in the future at current levels of fishing effort if a stock-recruitment relationship is assumed, particularly for the surface fisheries (Figure D-9).

Figure D-9: Historic and projected annual catches of bigeye tuna by the surface (top panel) and longline (bottom panel) fisheries from the base case while fishing with the current effort, the base case while fishing at the fishing mortality corresponding to MSY (F[msy]), and the analysis of sensitivity to steepness (h = 0.75) of the stock-recruitment relationship while fishing with the current effort. The large dot indicates the most recent observed catch (2011).

These simulations are based on the assumption that selectivity and catchability patterns will not change in the future. Changes in targeting practices or increasing catchability of bigeye as abundance declines (e.g. density-dependent catchability) could result in differences from the outcomes predicted here.