North Sea-focused oil and gas firm Ithaca Energy said it strengthened its business in the first half but swung to a loss before tax of $2.5m, compared with a profit of $73m the year before.The AIM-listed group reported $199.6m in revenue for the year ended June, slightly higher than last year's $188.1m, as the realised oil price remained relatively stable and pro-forma production rose 15% year-on-year.However, higher depletion, depreciation and amortisation costs and a prior-year goodwill benefit of $55.3m saw the bottom line sink. Earnings per share fell to five cents a share, down from 20 cents previously.After tax however, a deferred tax credit of $19.5m helped the firm to generate a profit of $17.0m for the first half, though this was down from $57.3m the previous year.Average pro-forma production in the first half of 2014 was approximately 12,800 barrels of oil equivalent per day including around 2,300 boepd from assets acquired from Sumitomo Corporation earlier in the year.Chief executive officer Les Thomas said: "I am pleased with the progress made by the company over the last quarter, having further strengthened the business in three key areas: the successful third Stella well materially de-risked the on-going field development; the Summit acquisition added high quality assets to the existing producing portfolio; and the successful bond offering introduced important funding diversification and flexibility."The stock was down 1.8% at 134p by 08:09.WS