Sunday, August 10, 2008

Number crunching exercise for gillnet impacts

In an earlier post we had estimated the Area 4 Skeena gillnet impact/encounters. In this post we've added the few last openings to come up with a season long tally.
If you want the short version: we figured 18,120 steelhead encountered and at 60% mortality this equals 10,800. At 70% mortality it equals 12,600.
And while the Tyee Index on Aug.04 was 64.23, we estimate it could have been 138.18 with the 'Inferred' Tyee Index numbers added in.

Please try to follow along in this math exercise where we calculate what the stimated number of steelhead killed by gillnetters would have added to the Tyee Index numbers. We call these the "Inferred Index Points" and the "Adjusted Index" is the regular Tyee Index number with our "Inferred" numbers added to it.
I'm sure the DFO scientific types can find a gazillion holes in this type of non-professional accounting, but there is alot of value in examining the encounter rates overall and attempting to put a number on it.
(Apologies for the long post.)

Date

# of Boats

Approx# of stld enc/day/
boat

Total stld enct

Infer. Index

Points

2008 Tyee Index

Adjust. Tyee Index

July 07

91

2

182

.74

.66

1.40

July 11

96

2

182

.78

1.32

2.10

July 15

282

3

846

3.45

1.71

5.16

July 17

300

3

900

3.67

.31

3.98

July 18

300

4

1200

4.90

.64

5.54

July 21

296

5

1480

6.04

1.96

8.0

July 22

314

8

2512

10.25

1.55

11.80

July 24

316

8

2528

10.31

2.13

12.44

July 27

241

8

1928

7.87

1.19

9.06

July 28

216

8

1728

7.05

4.66

11.71

July 31

289

8

2312

9.44

4.69

14.13

Aug. 04

289

4

1156

4.71

3.64

8.35

Aug. 07

289

4

1156

4.71

4.03

8.74

Total



18,120

73.95








Tyee Index on Aug.04: 64.23..............Adjusted Tyee Index with Impacted Stld #: 138.18

Our rough estimate for total potential steelhead encounters by gillnetters in Area 4 of 18,120 up to Aug.07.

If the Tyee multiplier ( 1 Index basis point equals 245 stld) for steelhead is 245....then 18,120 divided by 245 = 73.95 Index basis points

The Index on Aug. 7th was at 64.23. Add in the 'inferred' Index basis points of 73.95 = 138.18

This would put 2008 only behind 1998 @ 159.89........As the second highest on record up to that date. Remember, 1998 had no commercial fishing so can be seen as a 'clean run' to compare against....except for Alaskan impacts of course.

Now you could get picky and say not all 18K stld encountered by gillnets in Area 4 died:
Range= @ 60% mortality on 18K = 10,800 = 44.08 Index points + 2008 @ 64.23= 108.31 comes in 2nd place for that date

@ 70% mortality on 18K = 12,600 = 51.45 Index points + 2008 @ 64.23 = 115.70 also comes in 2nd place for that date

This little exercise is valuable for highlighting several issues with the commercial fishery for Skeena steelhead. Most obviously it reinforces the degree to which the Area 4 gillnet fishery negatively impacts on the steelhead run, especially the early component.

Moreover, it highlights the fact that over half of the steelhead run to that point encounters a gillnet. i.e if the Index equals 64.23 ( as of Aug.4th) x 245 stld multiplier= 15,736 stld past Tyee estimated. We also estimated the encounter number for steelhead with gillnetters in Area 4 at 18,120 up to Aug.7. So, from our rough estimates we can see that over 50% of the run encountered a gillnet.

High encounter rates are an important issue as the impact in a low abundance year could be quite severe leading to true conservation concerns. And again, most of this commercial impact is disproportionately placed on the early component of the steelhead run. These early returning steelhead are the most valuable to the upriver sportfishery as they arrive the earliest and stay the longest.
Please note in our exercise we didnt include impacts from the seine fishery in Area 4 or gillnet /seine impacts from the Area 3 fishery, which would obviously add to the encounter numbers we estimated.

4 comments:

kent brocker said...

I've fished the skeena system for twenty years,sorry I just found you.I,m not sure of the parameters followed by the commercial fishery but something better has to be proposed to protect this WORLD
TREASURE! I know the solution but that is highly unlikely. How about escapement days netting for 5 out of 7 days but no increase in allowed days? Just an idea. Anyway,I hope the data foretells the angling success,we can only hope.Thankyou, joining the alliance this minute.

deanbc said...

Your number crunching exercise is a good one and it begs several questions. If we use this type of data to estimate potential negative impact, we (including the DFO)can and should be able to come up with some sort of compromise plan to work out a better overall process and plan, isn't that true?. One simple example that could be employed in 2009 is to allow gill netting every other day, say during a 20 to 22 day season...then do the same type of calculations as you did with your current post. This would allow Steelhead and other non-targeted fish an opportunity to move freely upriver with out net intervention. Then data can be compared to estimate the size of the run for one year compared to the estimmated size the next year and then I am sure things can be figured out resulting in a total impact number for that netting process as you attempt to determine impact by using your number crunching example.

In other words, why can't we try a few things that can result in compromise for both the Commercial and sport fisherman and see what happens? If it gives the Commercial guys access to get what they are looking for and it allows more non-targeted fish to move upriver, why would anyone be against trying several approaches, including the DFO?

Of Course, alternative netting procedures could be tested the same way. For example, tell the fleet now that for 2009 there will be only short set seine netting allowed and again when done, compare the data. The point is, we know stocks are threatened, nobody including the DFO can argue with this so lets try a few things to come up with a better process that compromises for all.

North Coast Steelhead Alliance said...

kent; thanks for joining.
we think the key is getting away from non-selective gillnets altogether....or at the very least using tangle nets in very restricted areas and times.

you just cant allow 300 gillnetters to impact non-target stocks like they do anymore...the non-target fish, especially steelhead are too important.

Greg said...

As time allows:
Could you possibly prepare a Google Earth map and show us approximately where 300(!) Gill net sets do occur?
For those of us who fish up river I think it would be helpful/alarming to see how inclusive the sets really are and how likely impossible it is for many fish to escape impacts.

Greg