XL Catlin’s net income for the fourth quarter of 2015 jumped 63.8% over the prior-year period to $228.6 million despite natural catastrophe losses and integration expenses from the merger of XL Group P.L.C. and Catlin Group Ltd.

Revenue for the quarter climbed 81.8% to $2.63 billion as net written premiums increased 54.6% to $1.89 billion and net investment income dropped a scant 0.2% to $171.9 million, the company said Wednesday in its earnings release.

Expenses for the quarter rose 99.7% to $2.44 billion on natural catastrophe losses of $107.8 million — more than triple those of the fourth quarter of 2014 — and integration costs of $73.3 million.

The company’s combined ratio for the quarter deteriorated to 92.3% from 84.5% in the year-ago period.

The $4.1 billion merger of XL and Catlin was concluded last May, resulting in the formation of XL Catlin.

For the full year, revenue rose 41.0% to $9.31 billion as net written premiums increased 33.7% to $7.95 billion and net investment income declined 13.2% to $684.9 million.

Expenses for the year grew 32.4% to $8.40 billion as natural catastrophe losses jumped 88.2% to $213.2 million and integration costs reached $156.4 million.

The company’s property/casualty combined ratio for the year deteriorated to 92% from 88.2% in the prior year.

In a research note, Cliff Gallant, managing director at San-Francisco-based Nomura Securities International Inc., said both reserve releases of $121 million and catastrophe losses of $108 million were higher than estimates of $60 million and $30 million, respectively.

On a positive note, Mr. Gallant said “the Catlin merger remains on track, and we continue to expect improving (returns on equity) through 2017.”

“We believe there is a huge payoff for having gone first, for having carefully picked out our partner, and for having done all this work rapidly and efficiently,” said CEO Mike McGavick on a Thursday earnings call with analysts.

He said the company has now identified $300 million in potential synergy savings, up from initial estimates of $200 million “as the transaction continues to reveal additional savings opportunities.”

Mr. McGavick added that the company believes it has lost less than 0.5% of premium due to the merger, less than a tenth of what was originally modeled. This will grow to approximately 1% when a program matures in the second quarter, he said.